Ignore broken symlinks and outside path, in commit

[squaremo]

There is a bug in go-git which leads to it reporting broken, absolute
symlinks as modified whether they are or not:

https:/go-git/go-git/issues/253

To date, the controller checks whether the repo it has run an update
on is Clean, and as a consequence will run into the bug above if a
broken symlink is in the repo. The result is that it makes and pushes
an empty commit every interval.

To work around the problem, this commit adds a more careful check of
the repo status. Each file reported as modified is validated, firstly
by checking that it's under the path given; and secondly by checking
specifically that it's not a broken symlink.

For convenience, I have moved a few procedures around so they can be
used more readily by go tests.

Fixes #135.

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firstly by checking that it's under the path given

Of this bit I am not convinced. It's possible you have a valid symlink under the path, targeting a file not under the path, and you want the update to work. It's also possible you've come to this arrangement by accident -- but I shouldn't presume so!

UPDATE: I removed the path limitation in another commit; it can be squashed in, or reset out.

[stefanprodan]

LGTM

Thanks @squaremo 🥇

@hiddeco please incorporate the symlink check in fluxcd/flux2#968

[squaremo]

I'm going to squash in the commit removing the path limitation, then merge.