[gcc r11-1193] prepare-commit-hook: Use gcc-config.diff-file.

[Martin Liska]

https://gcc.gnu.org/g:8ff57396d6dea4976b05902919070aa0e562354d

commit r11-1193-g8ff57396d6dea4976b05902919070aa0e562354d
Author: Martin Liska <mliska@suse.cz>
Date:   Thu Jun 11 10:02:26 2020 +0200

    prepare-commit-hook: Use gcc-config.diff-file.
    
    contrib/ChangeLog:
    
            * prepare-commit-msg: Replace ENV variable with a git config
            value.

Diff:
---
 contrib/prepare-commit-msg | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/contrib/prepare-commit-msg b/contrib/prepare-commit-msg
index 57bb91747f6..cc9ba2e6ba1 100755
--- a/contrib/prepare-commit-msg
+++ b/contrib/prepare-commit-msg
@@ -58,8 +58,9 @@ else
 fi
 
 # Save diff to a file if requested.
-if ! [ -z "$GCC_GIT_DIFF_FILE" ]; then
-    tee="tee $GCC_GIT_DIFF_FILE"
+DIFF_FILE=$(git config gcc-config.diff-file)
+if ! [ -z "$DIFF_FILE" ]; then
+    tee="tee $DIFF_FILE"
 else
     tee="cat"
 fi