We do not like "HEAD" as a new branch name

This makes git-check-ref-format fail for "HEAD". Since the check is only
executed when creating refs, the existing symbolic ref is safe.

Otherwise these commands, most likely are pilot errors, would do
pretty funky stuff:

	git checkout -b HEAD
	git pull . other:HEAD
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

refs.c

@@ -345,6 +345,11 @@ int check_ref_format(const char *ref)
 		if (!ch) {
 			if (level < 2)
 				return -1; /* at least of form "heads/blah" */
+
+			/* do not allow ref name to end in "HEAD" */
+			if (cp - ref > 4 && !strcmp(cp - 4, "HEAD"))
+				return -1;
+
 			return 0;
 		}
 	}