commit-msg: Preserve bad message for user to fix

Store a backup copy of the input message.  Remove the backup only on
success.  On failure, tell the user how to use and edit the message
without having to enter it from scratch.

commit-msg

@@ -2,11 +2,18 @@
 #
 # Copy or link this file as ".git/hooks/pre-commit".
 
+# Prepare a backup message without comments.
+commit_msg="$GIT_DIR/COMMIT_MSG"
+grep -v '^#' "$1" > "$commit_msg"
+
 die() {
 	echo 'commit-msg hook failure' 1>&2
 	echo '-----------------------' 1>&2
 	echo '' 1>&2
 	echo "$@" 1>&2
+	echo 'To continue editing, run the command
+  git commit -e -F '"$commit_msg"'
+(assuming your working directory is at the top).' 1>&2
 	exit 1
 }
 
@@ -51,7 +58,8 @@ msg_rest() {
 
 # Pipe commit message into the state machine.
 state=first
-cat "$1" | grep -v '^#' |
+cat "$commit_msg" |
 while read line; do
 	msg_$state || break
-done
+done &&
+rm -f "$commit_msg"