contrib: Make prepare-commit-msg hook smarter for amends

With this change the prepare-commit-msg hook can compare the log of a
commit being amended with the staged changes, and not run mklog.py
unnecessarily. This is controlled by a git config option,
gcc-config.mklog-hook-type.

contrib/ChangeLog:

	* prepare-commit-msg: Use the gcc-config.mklog-hook-type Git
	config key instead of the GCC_FORCE_MKLOG environment variable.
	Optionally disable generating a new ChangeLog template for
	amended commits when the existing log is still OK.

contrib/prepare-commit-msg

@@ -49,6 +49,19 @@ elif [ $COMMIT_SOURCE = commit ]; then
     # otherwise, assume a new commit with -C.
     if [ $SHA1 = HEAD ]; then
 	cmd="diff --cached HEAD^"
+	if [ "$(git config gcc-config.mklog-hook-type)" = "smart-amend" ]; then
+	    # Check if the existing message still describes the staged changes.
+	    f=$(mktemp /tmp/git-commit.XXXXXX) || exit 1
+	    git log -1 --pretty=email HEAD > $f
+	    printf '\n---\n\n' >> $f
+	    git $cmd >> $f
+	    if contrib/gcc-changelog/git_email.py "$f" >/dev/null 2>&1; then
+		# Existing commit message is still OK for amended commit.
+		rm $f
+		exit 0
+	    fi
+	    rm $f
+	fi
     else
 	cmd="diff --cached"
     fi