util/gitutil: fix TestGitFullCommitErr/TestGitShortCommitErr

git rev-parse HEAD doesn't need any `--`, however, when it is called
on an empty repo git returns an error:

```
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
```

So, it is both IsUnknownRevision and IsAmbiguousArgument.

Lets simply fix the test by flipping the check: I don't see
any dependency on this behavior in non-test code.

Author: jegorbunov

util/gitutil/gitutil_test.go

@@ -62,7 +62,7 @@ func TestGitFullCommitErr(t *testing.T) {
 	_, err = c.FullCommit()
 	require.Error(t, err)
 	require.True(t, gitutil.IsUnknownRevision(err))
-	require.False(t, gittestutil.IsAmbiguousArgument(err))
+	require.True(t, gittestutil.IsAmbiguousArgument(err))
 }
 
 func TestGitShortCommitErr(t *testing.T) {
@@ -75,7 +75,7 @@ func TestGitShortCommitErr(t *testing.T) {
 	_, err = c.ShortCommit()
 	require.Error(t, err)
 	require.True(t, gitutil.IsUnknownRevision(err))
-	require.False(t, gittestutil.IsAmbiguousArgument(err))
+	require.True(t, gittestutil.IsAmbiguousArgument(err))
 }
 
 func TestGitTagsPointsAt(t *testing.T) {