User visible paths have alien directory separator in the git module

All over in in the Git plugin, the (Windows) user gets to read paths with wrong directory delimiters. Qt Creator wold look less alien if all paths shown to the users got the QDir::toNativeSeparators treatment, before.

commit 679b5e91f13651d29abf1119ec473ced1ab0943c does 3% of that fixing. Ideally, someone who knows that code would search for all 'errorMessage' and 'workingDirectory' occurences in gitclient.cpp and insert QDir::toNativeSeparators when it makes sense.