teach mklog to look in the current directory for ChangeLog files

teach mklog to look in the current directory for ChangeLog files

 when run in repos other than gcc mklog fails to find ChangeLog files
because it looks for $0/../$dir/ChangeLog, but of course if the diff is
for a project other than gcc that might not exist.  It should be fine to
also look for $cwd/$dir/ChangeLog, and use that if we find it.  This
means that for example in binutils-gdb.git you can do git commit,
and then in your editor read git diff HEAD~ | mklog - to generate a
template ChangeLog for that commit.

contrib/ChangeLog:

2016-03-06  Trevor Saunders  <tbsaunde+gcc@tbsaunde.org>

	* mklog: Look for the ChangeLog file in $cwd.

From-SVN: r234014