Is this _complete() necessary?

It is not.  Though _complete won't actually change the buffer if the
trailing character matches, I've removed it.  The '/' is added prior
to each call here [1].

[1]: https://github.com/git/git/blob/277cf0bc36094f6dc4297d8c9cef79df045b735d/dir.c#L2329

This function is repeatedly called during the traversal.

How expensive is it to keep calling real_pathdup() on the constant
the_repository->gitdir just in case it might be the same as our true
GIT_DIR?

Using trace2, `real_pathdup` takes around 40 usecs on my machine
compared to 10 usecs for an empty region.

Please see the cover letter for v3 of the patch series for additional
details on both of these items.