WIP: use cygpath.exe to convert Windows paths and respect mount-points

[ankostis]

Currently the /cygdrive/ mount-point is hard-coded in GitPython's conversions git/utils.py since version 2.1.3 that Cygwin support was added. This PR purports to use Cygwin's cygpath -w | -u utility for path conversions.
An added benefit is support out-of-the-box for MSYS2 which is a Cygwin clone based on the excellent pacman package manager that (at last) mounts Windows-drives under root(/).

It is still a WorkInProgress (WIP) because although all travis TCs pass on the POSIX part, appveyor now fails on all non-Cygwin builds; prior to this PR, it was only Cygwin builds that were failing in a couple of TCs: https://ci.appveyor.com/project/ankostis/gitpython/build/1.0.443).

[codecov-io]

Codecov Report

Merging #639 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #639 +/- ## ========================================== - Coverage 92.57% 92.48% -0.09%  ========================================== Files 61 61 Lines 9968 9985 +17 ========================================== + Hits 9228 9235 +7 - Misses 740 750 +10

Impacted Files	Coverage Δ
git/objects/submodule/util.py	75.47% <0%> (-1.72%)	⬇️
git/test/test_repo.py	95.1% <0%> (+0.33%)	⬆️

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[Byron]

@ankostis I happen to be on a merge-spree right now and wonder if this one is ready. Also I know you could just merge yourself when ready, I am just wondering :). Thank you.

[ankostis]

Sorry, I cannot respond confidently.
I have imported this as hotfix in my code without any side-effect working on MSYS2 & Cygwin (less confident) for the last 2-3 months.

But still not all TCs pass, but have no time to fix them.
I would wait for them to be fixed; we already have 2 "worktree" TCs failing in Cygwin, so better not make it any worse than that.

Apologies for me not being available.