Fix upstream.sh presubmit test for git 1.8.3+

Previously this relied on being able to track branches which didn't have a
remote refspec. Now we add a fake origin for the git-svn test repo, and use
--prefix so that git has a remote to track and the refs are correctly
namespaced.

Should also be backwards compatible with older versions of git.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/61893006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@233649 0039d316-1c4b-4281-b951-d872f2087c98

tests/test-lib.sh

@@ -49,9 +49,10 @@ setup_gitsvn() {
   rm -rf git-svn
   # There appears to be no way to make git-svn completely shut up, so we
   # redirect its output.
-  git svn -q clone -s $REPO_URL git-svn >/dev/null 2>&1
+  git svn --prefix origin/ -q clone -s $REPO_URL git-svn >/dev/null 2>&1
   (
     cd git-svn
+    git remote add origin https://example.com/fake_refspec
     git config user.name 'TestDood'
     git config user.email 'TestDood@example.com'
   )

tests/upstream.sh

@@ -19,7 +19,7 @@ setup_gitsvn
 
   git config rietveld.server localhost:8080
 
-  for ref in refs/remotes/trunk refs/remotes/some_branch; do
+  for ref in refs/remotes/origin/trunk refs/remotes/origin/some_branch; do
     git branch -f --set-upstream feature_branch $ref
     git checkout -q feature_branch
     test_expect_success "Guessing upstream branch for $ref" \