RFC: Better commit messages for libstdc++

[Jonathan Wakely <jwakely at redhat dot com>]

With the conversion from Subversion to Git now scheduled for the end
of this year (see https://gcc.gnu.org/wiki/GitConversion for the
plans) I think it's time to consider a policy for better commit
messages.

I want all commits to libstdc++ to include a one-line summary message,
with a blank line before the rest of the message. This should be a
hard requirement. We can do this now, we don't need to wait until we
stop using Subversion. Ideally this would also be adopted for the rest
of GCC, but I'll start with libstdc++.

I suggest reading https://chris.beams.io/posts/git-commit/ if you
don't already agree :-)

Currently the output of 'git log --oneline' is often unreadable.
There are lots of entries which just past the ChangeLog entry, so the
first line is just the date and committer's name and email address.
That's not useful as a summary of the commit. Other commits show the
list of changed files, which is too detailed, a summary would be
better.

2d9c100659d Implement LWG 3255 for std::span constructors
047fcaf1e11 PR libstdc++/77936 remove unused variable
c6fd1c4cf03 [PR 91853] Prevent IPA-SRA ICEs on type-mismatched calls
9ce1ca72088 2019-09-30  Andreas Tobler  <andreast@gcc.gnu.org>
70f0835d2a9 	PR target/91931 	* config/i386/i386-expand.c (ix86_expand_adjust_ufix_to_sfix_si): Use 	gen_int_mode instead of GEN_INT.
ed76597323f Daily bump.
ef6064d4ff4 2019-09-29  Steven G. Kargl  <kargl@gcc.gnu.org>
0c7c01cf936 [Darwin, PPC, Mode Iterators 5/n] Update macho_low.
a3310563550 2019-09-29  Steven G. Kargl  <kargl@gcc.gnu.org>
e020479cd65 2019-09-29  Paul Thomas  <pault@gcc.gnu.org>
e9eb6ea5605 	PR bootstrap/90543 	* optc-save-gen.awk: Fix up printing string option differences.
56f22142aba gcc/ChangeLog
49f75e008c0 This patch is to add the support for float from/to long conversion vectorization on port rs6000.
8ee0996558f 2019-09-28  Jerry DeLisle  <jvdelisle@gcc.ngu.org>
1cbc7e9bf01 Daily bump.
a4fcce58dff 2019-09-28  François Dumont  <fdumont@gcc.gnu.org>
1334eb86c38 [Darwin, PPC, Mode Iterators 4/n] Update macho_high.
cb9a5db89bd 2019-09-28  Jerry DeLisle  <jvdelisle@gcc.gnu.org>
9587be52124 2019-09-28  Steven G. Kargl  <kargl@gcc.ngu.org>
c6331a1f238 2019-09-28  Steven G. Kargl  <kargl@gcc.gnu.org>
aeca84f4c1e 	PR c++/91889 - follow-up fix for DR 2352. 	* call.c (involves_qualification_conversion_p): New function. 	(direct_reference_binding): Build a ck_qual if the conversion 	would involve a qualification conversion. 	(convert_like_real): Strip the conversion created by the ck_qual 	in direct_reference_binding.
e95722140ec 	PR c++/91921 - stray warning with -Woverloaded-virtual. 	* class.c (warn_hidden): Only emit the second part of 	-Woverloaded-virtual if the first part was issued.  Use inform instead 	warning_at.
6d88a6b8c4f 	PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17. 	* pt.c (invalid_nontype_parm_type_p): Only emit errors when 	tf_error.
492c602a810 gcc/ 2019-09-28  Oleg Endo  <olegendo@gcc.gnu.org>
67e0527b1bd gcc/ 2019-09-28  Oleg Endo  <olegendo@gcc.gnu.org>
dcfd68ec29a Fix endian issue in pr91656 testcases
f840368602b     compiler: resolve importing ambiguity for more complex function calls
1eedf33ccdc Daily bump.
a8864f4a61a Regenerate `configure' scripts for `uclinuxfdpiceabi' libtool.m4 update
853517aff15 	PR c++/88203 c-family/ 	* c-common.h (c_omp_predefined_variable): Declare. 	* c-omp.c (c_omp_predefined_variable): New function. 	(c_omp_predetermined_sharing): Return OMP_CLAUSE_DEFAULT_SHARED 	for predefined variables. c/ 	* c-parser.c (c_parser_predefined_identifier): New function. 	(c_parser_postfix_expression): Use it. 	(c_parser_omp_variable_list): Parse predefined identifiers. 	* c-typeck.c (c_finish_omp_clauses): Allow predefined variables 	in shared and firstprivate clauses, even when they are predetermined 	shared. cp/ 	* parser.c (cp_parser_omp_var_list_no_open): Parse predefined 	variables. 	* semantics.c (finish_omp_clauses): Allow predefined variables in 	shared and firstprivate clauses, even when they are predetermined 	shared. 	* cp-gimplify.c (cxx_omp_predetermined_sharing_1): Return 	OMP_CLAUSE_DEFAULT_SHARED for predefined variables. testsuite/ 	* c-c++-common/gomp/pr88203-1.c: New test. 	* c-c++-common/gomp/pr88203-2.c: New test. 	* c-c++-common/gomp/pr88203-3.c: New test.

This last one is particularly bad!

Take a look at 'git log --oneline --author=redi' for how it should
look:

2d9c100659d Implement LWG 3255 for std::span constructors
047fcaf1e11 PR libstdc++/77936 remove unused variable
29a4c7d93a6 PR libstdc++/91910 fix data race in Debug Mode destructors
563af6e4987 Define std::to_array for Debug Mode
f875c1ba2b0 Implement C++20 constexpr changes to std::pair (P1032R1)
b860dbe9791 Fix array index error in address_v6 comparisons
d00ba20dbd7 Remove include directives for deleted Profile Mode headers
17404fb2aa2 Implement LWG 3296 for basic_regex::assign
3efa913a6e4 Remove check for impossible condition in std::variant::index()
4c6edff3676 PR libstdc++/91871 fix Clang warnings in testsuite
de61fb434a5 PR libstdc++/91788 improve codegen for std::variant<T...>::index()
c3ae6f9b28e PR libstdc++/91748 fix std::for_each_n for random access iterators
0f5caaa9b5f Fix Xmethod for shared_ptr::use_count()
25f120f074c Fix libstdc++ tests for -Wvolatile warnings in C++2a mode
e4c0b7eac16 Implement std::common_reference for C++20


The format I'd like to see for libstdc++ commits is shown below. If
the change is fixing a bug or implementing a WG21 proposal or issue,
that should be mentioned in the summary. Typically, the summary would
make a good email Subject: line, and the paragraphs following the
summary would make a good email body describing the patch (everybody
is already writing an email Subject and an email body, so there is no
extra work needed to reuse t hat text in the commit message). The
ChangeLog entry should be provided at the end, just like it is today.
The only difference is that a summary line is required before the
changelog text, and a descriptive body is encouraged.


Proposed format:

PR libstdc++/NNNNN one-line summary of the change

Optional additional text describing the change here. You can use this
to describe the problem being solved, and how the patch addresses it.
This can be as long or short as you like, in multiple paragraphs if
you want (or omitted entirely if there's nothing useful that isn't
already said by the summary line and the changelog).

   YYYY-MM-DD  John Doe  <jdoe@example.com>

   * foobar.cc: standard GNU changelog at the end.