Clear error when rebase has broken parent reference

[OskiKervinen-MF]

I had rebased the commit where I performed the git subrepo clone and when I tried to do a pullafter that, I received the error

git-subrepo: Command failed: 'git branch subrepo/utilities/cacherer '.
fatal: Not a valid object name: ''.

I was able to figure out this was because of the rebase eventually, but it took some doing, so I made this MR to make the problem obvious in the future.

Also replaced one raw reference to .git with an invocation of git rev-parse --git-dir.

[admorgan]

I like the error message change, but https://github.com/ingydotnet/git-subrepo/pull/407 does a better job fixing the submodule issue.

[OskiKervinen-MF]

I removed the part that touched the submodule issue. The remaining test failures seem to equal those in master.

[arichardson]

I've accidentally rebased after doing a subrepo push/pull a few times and then getting weird failures the next time I try to update. Having a meaningful error message would be really useful. Thanks!

[taktran]

I found a potential fix for this issue here: https://github.com/ingydotnet/git-subrepo/issues/617