branch: give advice when tracking start-point is missing

If the user requests to --set-upstream-to a branch that does
not exist, then either:

  1. It was a typo.

  2. They thought the branch should exist.

In case (1), there is not much we can do beyond showing the
name we tried to use. For case (2), though, we can help to
guide them through common workflows.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

advice.c

@@ -13,6 +13,7 @@ int advice_commit_before_merge = 1;
 int advice_resolve_conflict = 1;
 int advice_implicit_identity = 1;
 int advice_detached_head = 1;
+int advice_set_upstream_failure = 1;
 
 static struct {
 	const char *name;
@@ -31,6 +32,7 @@ static struct {
 	{ "resolveconflict", &advice_resolve_conflict },
 	{ "implicitidentity", &advice_implicit_identity },
 	{ "detachedhead", &advice_detached_head },
+	{ "setupstreamfailure", &advice_set_upstream_failure },
 
 	/* make this an alias for backward compatibility */
 	{ "pushnonfastforward", &advice_push_update_rejected }

advice.h

@@ -16,6 +16,7 @@ extern int advice_commit_before_merge;
 extern int advice_resolve_conflict;
 extern int advice_implicit_identity;
 extern int advice_detached_head;
+extern int advice_set_upstream_failure;
 
 int git_default_advice_config(const char *var, const char *value);
 void advise(const char *advice, ...);

branch.c

@@ -200,7 +200,16 @@ int validate_new_branchname(const char *name, struct strbuf *ref,
 static const char upstream_not_branch[] =
 N_("Cannot setup tracking information; starting point '%s' is not a branch.");
 static const char upstream_missing[] =
-N_("Cannot setup tracking information; starting point '%s' does not exist");
+N_("the requested upstream branch '%s' does not exist");
+static const char upstream_advice[] =
+N_("\n"
+"If you are planning on basing your work on an upstream\n"
+"branch that already exists at the remote, you may need to\n"
+"run \"git fetch\" to retrieve it.\n"
+"\n"
+"If you are planning to push out a new local branch that\n"
+"will track its remote counterpart, you may want to use\n"
+"\"git push -u\" to set the upstream config as you push.");
 
 void create_branch(const char *head,
 		   const char *name, const char *start_name,
@@ -230,8 +239,14 @@ void create_branch(const char *head,
 
 	real_ref = NULL;
 	if (get_sha1(start_name, sha1)) {
-		if (explicit_tracking)
+		if (explicit_tracking) {
+			if (advice_set_upstream_failure) {
+				error(_(upstream_missing), start_name);
+				advise(_(upstream_advice));
+				exit(1);
+			}
 			die(_(upstream_missing), start_name);
+		}
 		die("Not a valid object name: '%s'.", start_name);
 	}