get_sha1_basic(): try refs/... and finally refs/remotes/$foo/HEAD

This implements the suggestion by Jeff King to use
refs/remotes/$foo/HEAD to interpret a shorthand "$foo" to mean
the primary branch head of a tracked remote.  clone needs to be
told about this convention as well.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

sha1_name.c

@@ -235,18 +235,21 @@ static int ambiguous_path(const char *path, int len)
 
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
-	static const char *prefix[] = {
-		"",
-		"refs",
-		"refs/tags",
-		"refs/heads",
-		"refs/remotes",
+	static const char *fmt[] = {
+		"/%.*s",
+		"refs/%.*s",
+		"refs/tags/%.*s",
+		"refs/heads/%.*s",
+		"refs/remotes/%.*s",
+		"refs/remotes/%.*s/HEAD",
 		NULL
 	};
 	const char **p;
 	const char *warning = "warning: refname '%.*s' is ambiguous.\n";
 	char *pathname;
 	int already_found = 0;
+	unsigned char *this_result;
+	unsigned char sha1_from_ref[20];
 
 	if (len == 40 && !get_sha1_hex(str, sha1))
 		return 0;
@@ -255,11 +258,9 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	if (ambiguous_path(str, len))
 		return -1;
 
-	for (p = prefix; *p; p++) {
-		unsigned char sha1_from_ref[20];
-		unsigned char *this_result =
-			already_found ? sha1_from_ref : sha1;
-		pathname = git_path("%s/%.*s", *p, len, str);
+	for (p = fmt; *p; p++) {
+		this_result = already_found ? sha1_from_ref : sha1;
+		pathname = git_path(*p, len, str);
 		if (!read_ref(pathname, this_result)) {
 			if (warn_ambiguous_refs) {
 				if (already_found &&