branch: improve error message for missing --set-upstream-to ref

If we are trying to set the upstream config for a branch,
the create_branch function will check both that the name
resolves as a ref, and that it is either a local or
remote-tracking branch.

However, before we do so we run get_sha1 on it to find out
whether it resolves at all (since the create_branch function
is also used to create actual branches, it wants to know
where to start the new branch). This means that if you feed
a ref that does not exist to "branch --set-upstream-to",
rather than getting a helpful message about tracking, you
only get "not a valid object name".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

branch.c

@@ -199,6 +199,8 @@ int validate_new_branchname(const char *name, struct strbuf *ref,
 
 static const char upstream_not_branch[] =
 N_("Cannot setup tracking information; starting point is not a branch.");
+static const char upstream_missing[] =
+N_("Cannot setup tracking information; starting point does not exist");
 
 void create_branch(const char *head,
 		   const char *name, const char *start_name,
@@ -227,8 +229,11 @@ void create_branch(const char *head,
 	}
 
 	real_ref = NULL;
-	if (get_sha1(start_name, sha1))
+	if (get_sha1(start_name, sha1)) {
+		if (explicit_tracking)
+			die(_(upstream_missing));
 		die("Not a valid object name: '%s'.", start_name);
+	}
 
 	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
 	case 0: