Be more strict with git arguments (#7715)

* Be more strict with git arguments
* fix-up commit test
* use bindings for branch name

modules/git/repo_commit_test.go

@@ -58,5 +58,5 @@ func TestGetCommitWithBadCommitID(t *testing.T) {
 	commit, err := bareRepo1.GetCommit("bad_branch")
 	assert.Nil(t, commit)
 	assert.Error(t, err)
-	assert.EqualError(t, err, "object does not exist [id: bad_branch, rel_path: ]")
+	assert.True(t, IsErrNotExist(err))
 }