prevent HEAD reflog to be interpreted as current branch reflog

The work in progress to enable separate reflog for HEAD will make it
independent from reflog of any branch HEAD might be pointing to. In
the mean time disallow HEAD@{...} until that work is completed. Otherwise
people might get used to the current behavior which makes HEAD@{...} an
alias for <current_branch>@{...} which won't be the case later.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

sha1_name.c

@@ -301,12 +301,26 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		fprintf(stderr, warning, len, str);
 
 	if (reflog_len) {
-		/* Is it asking for N-th entry, or approxidate? */
 		int nth, i;
 		unsigned long at_time;
 		unsigned long co_time;
 		int co_tz, co_cnt;
 
+		/*
+		 * We'll have an independent reflog for "HEAD" eventually
+		 * which won't be a synonym for the current branch reflog.
+		 * In the mean time prevent people from getting used to
+		 * such a synonym until the work is completed.
+		 */
+		if (!strncmp("HEAD", str, len) &&
+		    !strncmp(real_ref, "refs/", 5)) {
+			error("reflog for HEAD has not been implemented yet\n"
+			      "Maybe you could try %s%s instead.",
+			      strchr(real_ref+5, '/')+1, str + len);
+			exit(-1);
+		}
+
+		/* Is it asking for N-th entry, or approxidate? */
 		for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
 			char ch = str[at+2+i];
 			if ('0' <= ch && ch <= '9')