interpret_branch_name: avoid @{upstream} past colon

get_sha1() cannot currently parse a valid object name like
"HEAD:@{upstream}" (assuming that such an oddly named file
exists in the HEAD commit). It takes two passes to parse the
string:

  1. It first considers the whole thing as a ref, which
     results in looking for the upstream of "HEAD:".

  2. It finds the colon, parses "HEAD" as a tree-ish, and then
     finds the path "@{upstream}" in the tree.

For a path that looks like a normal reflog (e.g.,
"HEAD:@{yesterday}"), the first pass is a no-op. We try to
dwim_ref("HEAD:"), that returns zero refs, and we proceed
with colon-parsing.

For "HEAD:@{upstream}", though, the first pass ends up in
interpret_upstream_mark, which tries to find the branch
"HEAD:". When it sees that the branch does not exist, it
actually dies rather than returning an error to the caller.
As a result, we never make it to the second pass.

One obvious way of fixing this would be to teach
interpret_upstream_mark to simply report "no, this isn't an
upstream" in such a case. However, that would make the
error-reporting for legitimate upstream cases significantly
worse. Something like "bogus@{upstream}" would simply report
"unknown revision: bogus@{upstream}", while the current code
diagnoses a wide variety of possible misconfigurations (no
such branch, branch exists but does not have upstream, etc).

However, we can take advantage of the fact that a branch
name cannot contain a colon. Therefore even if we find an
upstream mark, any prefix with a colon must mean that
the upstream mark we found is actually a pathname, and
should be disregarded completely. This patch implements that
logic.

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

sha1_name.c

@@ -1095,6 +1095,9 @@ static int interpret_upstream_mark(const char *name, int namelen,
 	if (!len)
 		return -1;
 
+	if (memchr(name, ':', at))
+		return -1;
+
 	set_shortened_ref(buf, get_upstream_branch(name, at));
 	return len + at;
 }

t/t1507-rev-parse-upstream.sh

@@ -210,4 +210,20 @@ test_expect_success 'log -g other@{u}@{now}' '
 	test_cmp expect actual
 '
 
+test_expect_success '@{reflog}-parsing does not look beyond colon' '
+	echo content >@{yesterday} &&
+	git add @{yesterday} &&
+	git commit -m "funny reflog file" &&
+	git hash-object @{yesterday} >expect &&
+	git rev-parse HEAD:@{yesterday} >actual
+'
+
+test_expect_success '@{upstream}-parsing does not look beyond colon' '
+	echo content >@{upstream} &&
+	git add @{upstream} &&
+	git commit -m "funny upstream file" &&
+	git hash-object @{upstream} >expect &&
+	git rev-parse HEAD:@{upstream} >actual
+'
+
 test_done