[gcc/devel/c++-modules] prepare-commit-msg: Save diff if requested by user.

[Nathan Sidwell]

https://gcc.gnu.org/g:7b8ee33affd968d3a8f5f6224268702d1900f0a1

commit 7b8ee33affd968d3a8f5f6224268702d1900f0a1
Author: Martin Liska <mliska@suse.cz>
Date:   Tue May 26 09:08:31 2020 +0200

    prepare-commit-msg: Save diff if requested by user.
    
    contrib/ChangeLog:
    
            * prepare-commit-msg: Save diff to a file if
            the GCC_GIT_DIFF_FILE variable is set.

Diff:
---
 contrib/prepare-commit-msg | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/contrib/prepare-commit-msg b/contrib/prepare-commit-msg
index 820dacc7e42..fd59bfbcf88 100755
--- a/contrib/prepare-commit-msg
+++ b/contrib/prepare-commit-msg
@@ -54,4 +54,9 @@ else
     exit 0
 fi
 
+# Save diff to a file if requested.
+if ! [ -z "$GCC_GIT_DIFF_FILE" ]; then
+  git $cmd > "$GCC_GIT_DIFF_FILE";
+fi
+
 git $cmd | git gcc-mklog -c "$COMMIT_MSG_FILE"