Added CreateRemoteRef to repo_harness (and accompanying git package updates)

Added CreateRemoteRef to the repo_harness package as discussed in
crrev.com/c/1706930. Naturally, this required adding a few methods to
the git package.

TEST=run_tests.sh
BUG=chromium:980346

Change-Id: Ida29844381e3212fbe9b247af106c4dedffa7ed8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/1710949
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
diff --git a/internal/git/git_test.go b/internal/git/git_test.go
index 0587600..5b9202f 100644
--- a/internal/git/git_test.go
+++ b/internal/git/git_test.go
@@ -234,7 +234,7 @@
 	assert.NilError(t, err)
 }
 
-func TestPush(t *testing.T) {
+func TestPushRef(t *testing.T) {
 	fakeGitRepo := "repo"
 	localRef := "commitId"
 
@@ -249,7 +249,7 @@
 		ExpectedCmd: []string{"git", "push", remoteRef.Remote, pushStr, "--dry-run"},
 	}
 
-	err := Push(fakeGitRepo, localRef, true, remoteRef)
+	err := PushRef(fakeGitRepo, localRef, true, remoteRef)
 	assert.NilError(t, err)
 }