git subtree push to skip nested subrepos

[michelemartone]

Hi.
I am aware of #417 and #165 and #161 #82 and this ... is yet another query regarding 'nested subrepos' support :-)
Specifically, if both "bar" and "baz" in "bar/baz" are subrepos, then "git-subrepo push bar" pushes "baz" as well into the "bar" upstream.
Maybe it would make sense for "git-subrepo push" to skip directories containing ".gitrepo" (at least optionally)?

Background of this is to allow "git-subrepo" setups where one would lay subrepos according to the most natural order required by the packages, yet avoiding "excessive" pushes as described above.

[michelemartone]

I've put together a patch (#554) for push to skip nested subrepos, with (passing) test.
I omitted updating the documentation though; probably having this functionality as an option would be the best thing.