mingw: support UNC in git clone file://server/share/repo

mingw: support UNC in git clone file://server/share/repo

Extend the parser to accept file://server/share/repo in the way that
Windows users expect it to be parsed who are used to referring to file
shares by UNC paths of the form \\server\share\folder.

[jes: tightened check to avoid handling file://C:/some/path as a UNC
path.]

This closes https://github.com/git-for-windows/git/issues/1264.

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