Provide branch name in error message when using @{u}

When using @{u} or @{upstream} it is common to omit the branch name,
implying current branch. If the upstream is not configured, the error
message was "No upstream branch found for ''".

When resolving '@{u}', branch_get() is called, which almost always
returns a description of a branch. This allows us to use a branch name
in the error message, even if the user said something like '@{u}'.

The only case when branch_get() returns NULL is when HEAD points to so
something which is not a branch. Of course this also means that no
upstream is configured, but it is better to directly say that HEAD
does not point to a branch.

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

sha1_name.c

@@ -856,10 +856,14 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 	len = cp + tmp_len - name;
 	cp = xstrndup(name, cp - name);
 	upstream = branch_get(*cp ? cp : NULL);
-	if (!upstream
-	    || !upstream->merge
-	    || !upstream->merge[0]->dst)
-		return error("No upstream branch found for '%s'", cp);
+	/*
+	 * Upstream can be NULL only if cp refers to HEAD and HEAD
+	 * points to something different than a branch.
+	 */
+	if (!upstream)
+		return error("HEAD does not point to a branch");
+	if (!upstream->merge || !upstream->merge[0]->dst)
+		return error("No upstream branch found for '%s'", upstream->name);
 	free(cp);
 	cp = shorten_unambiguous_ref(upstream->merge[0]->dst, 0);
 	strbuf_reset(buf);

t/t1507-rev-parse-upstream.sh

@@ -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 ${sq}${sq}
+	error: No upstream branch found for ${sq}master${sq}
 	fatal: Needed a single revision
 	EOF
 	test_must_fail git rev-parse --verify @{u} 2>actual &&
@@ -152,7 +152,7 @@ test_expect_success 'branch@{u} error message with misspelt branch' '
 
 test_expect_success '@{u} error message when not on a branch' '
 	cat >expect <<-EOF &&
-	error: No upstream branch found for ${sq}${sq}
+	error: HEAD does not point to a branch
 	fatal: Needed a single revision
 	EOF
 	git checkout HEAD^0 &&