Windows: allow using UNC path for git repository

Windows: allow using UNC path for git repository

[efl: moved MinGW-specific part to compat/]
[jes: fixed compilation on non-Windows]

Eric Sunshine fixed mingw_offset_1st_component() to return
consistently "foo" for UNC "//machine/share/foo", cf

http://groups.google.com/group/msysgit/browse_thread/thread/c0af578549b5dda0

Author: Eric Sunshine <redacted>
Signed-off-by: Cezary Zawadka <redacted>
Signed-off-by: Eric Sunshine <redacted>
Signed-off-by: Erik Faye-Lund <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Stepan Kasal <redacted>
Signed-off-by: Junio C Hamano <redacted>