Be more specific if upstream branch is not tracked

If the branch configured as upstream didn't have a local tracking
branch, git said "Upstream branch not found". We can be more helpful,
and separate the cases when upstream is not configured, and when it is
configured, but the upstream branch is not tracked in a local branch.

The following configuration leads to the second scenario:

    [remote "origin"]
    	    url = ...
            fetch = refs/heads/master
    [branch "master"]
            remote = origin
            merge = refs/heads/master

'git pull' will work on master, but master@{upstream} is not defined.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -865,7 +865,12 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 	if (!upstream->merge || !upstream->merge[0]->dst) {
 		if (!ref_exists(upstream->refname))
 			return error("No such branch: '%s'", cp);
-		return error("No upstream branch found for '%s'", upstream->name);
+		if (!upstream->merge)
+			return error("No upstream configured for branch '%s'",
+				     upstream->name);
+		return error(
+			"Upstream branch '%s' not stored as a remote-tracking branch",
+			upstream->merge[0]->src);
 	}
 	free(cp);
 	cp = shorten_unambiguous_ref(upstream->merge[0]->dst, 0);

t/t1507-rev-parse-upstream.sh

@@ -125,7 +125,7 @@ test_expect_success 'branch@{u} works when tracking a local branch' '
 
 test_expect_success 'branch@{u} error message when no upstream' '
 	cat >expect <<-EOF &&
-	error: No upstream branch found for ${sq}non-tracking${sq}
+	error: No upstream configured for branch ${sq}non-tracking${sq}
 	fatal: Needed a single revision
 	EOF
 	error_message non-tracking@{u} 2>actual &&
@@ -134,7 +134,7 @@ test_expect_success 'branch@{u} error message when no upstream' '
 
 test_expect_success '@{u} error message when no upstream' '
 	cat >expect <<-EOF &&
-	error: No upstream branch found for ${sq}master${sq}
+	error: No upstream configured for branch ${sq}master${sq}
 	fatal: Needed a single revision
 	EOF
 	test_must_fail git rev-parse --verify @{u} 2>actual &&
@@ -162,7 +162,7 @@ test_expect_success '@{u} error message when not on a branch' '
 
 test_expect_success 'branch@{u} error message if upstream branch not fetched' '
 	cat >expect <<-EOF &&
-	error: No upstream branch found for ${sq}bad-upstream${sq}
+	error: Upstream branch ${sq}refs/heads/side${sq} not stored as a remote-tracking branch
 	fatal: Needed a single revision
 	EOF
 	error_message bad-upstream@{u} 2>actual &&