mingw: add a regression test for pushing to UNC paths

mingw: add a regression test for pushing to UNC paths

On Windows, there are "UNC paths" to access network (AKA shared)
folders, of the form \\server\sharename\directory. This provides a
convenient way for Windows developers to share their Git repositories
without having to have a dedicated server.

Git for Windows v2.11.0 introduced a regression where pushing to said
UNC paths no longer works, although fetching and cloning still does, as
reported here: https://github.com/git-for-windows/git/issues/979

This regression was fixed in 7814fbe3f1 (normalize_path_copy(): fix
pushing to //server/share/dir on Windows, 2016-12-14).

Let's make sure that it does not regress again, by introducing a test
that uses so-called "administrative shares": disk volumes are
automatically shared under certain circumstances, e.g.  the C: drive is
shared as \\localhost\c$. The test needs to be skipped if the current
directory is inaccessible via said administrative share, of course.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>