rebase: create HEAD reflog entry when aborting

When we abort a rebase, we return to the original value of
HEAD. Failing to write a reflog entry means we create a
gap in the reflog (which can cause "git show
HEAD@{5.minutes.ago}" to issue a warning). Plus having the
extra entry makes the reflog easier to follow for a human.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -332,7 +332,7 @@ abort)
 	read_basic_state
 	case "$head_name" in
 	refs/*)
-		git symbolic-ref HEAD $head_name ||
+		git symbolic-ref -m "rebase: aborting" HEAD $head_name ||
 		die "Could not move back to $head_name"
 		;;
 	esac