Fix commit-msg hook to allow editing

The old git-commit.sh script allowed the commit-msg hook to not only
prevent a commit from proceding, but also to edit the commit message
on the fly and allow it to proceed. So here we teach builtin-commit
to do the same.

This is based on Wincent's patch, but redone with a clarified logic.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -53,6 +53,8 @@ void launch_editor(const char *path, struct strbuf *buffer, const char *const *e
 			die("There was a problem with the editor %s.", editor);
 	}
 
+	if (!buffer)
+		return;
 	if (strbuf_read_file(buffer, path, 0) < 0)
 		die("could not read message file '%s': %s",
 		    path, strerror(errno));