branch: warn and refuse to set a branch as a tracking branch of itself.

Previous patch allows commands like "git branch --set-upstream foo foo",
which doesn't make much sense. Warn the user and don't change the
configuration in this case. Don't die to let the caller finish its job in
such case.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -49,9 +49,19 @@ static int should_setup_rebase(const char *origin)
 
 void install_branch_config(int flag, const char *local, const char *origin, const char *remote)
 {
+	const char *shortname = remote + 11;
+	int remote_is_branch = !prefixcmp(remote, "refs/heads/");
 	struct strbuf key = STRBUF_INIT;
 	int rebasing = should_setup_rebase(origin);
 
+	if (remote_is_branch
+	    && !strcmp(local, shortname)
+	    && !origin) {
+		warning("Not setting branch %s as its own upstream.",
+			local);
+		return;
+	}
+
 	strbuf_addf(&key, "branch.%s.remote", local);
 	git_config_set(key.buf, origin ? origin : ".");
 
@@ -71,8 +81,8 @@ void install_branch_config(int flag, const char *local, const char *origin, cons
 		strbuf_addstr(&key, origin ? "remote" : "local");
 
 		/* Are we tracking a proper "branch"? */
-		if (!prefixcmp(remote, "refs/heads/")) {
+		if (remote_is_branch) {
-			strbuf_addf(&key, " branch %s", remote + 11);
+			strbuf_addf(&key, " branch %s", shortname);
 			if (origin)
 				strbuf_addf(&key, " from %s", origin);
 		}