Fix minor bugs in vc-git and vc-hg on Windows uncovered by vc-tests

Fix minor bugs in vc-git and vc-hg on Windows uncovered by vc-tests

* lisp/vc/vc-hg.el (vc-hg-state-slow): Non-existing files emit a
different message on MS-Windows; support that.
* lisp/vc/vc-git.el (vc-git-checkin): Make sure
'default-directory' is not nil when calling
'make-nearby-temp-file' on MS-Windows.

* test/lisp/vc/vc-tests.el (vc-test--version-diff): Run
'default-directory' through 'file-truename', otherwise the
'vc-test-cvs06-version-diff' test might fail on MS-Windows.