fix pushing to //server/share/dir on Windows

normalize_path_copy() is not prepared to keep the double-slash of a
//server/share/dir kind of path, but treats it like a regular POSIX
style path and transforms it to /server/share/dir.

The bug manifests when 'git push //server/share/dir master' is run,
because tmp_objdir_add_as_alternate() uses the path in normalized
form when it registers the quarantine object database via
link_alt_odb_entries(). Needless to say that the directory cannot be
accessed using the wrongly normalized path.

Fix it by skipping all of the root part, not just a potential drive
prefix. offset_1st_component takes care of this, see the
implementation in compat/mingw.c::mingw_offset_1st_component().

This fixes https://github.com/git-for-windows/git/issues/979

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t5580-clone-push-unc.sh

@@ -34,7 +34,7 @@ test_expect_success clone '
 	git clone "file://$UNCPATH" clone
 '
 
-test_expect_failure push '
+test_expect_success push '
 	(
 		cd clone &&
 		git checkout -b to-push &&