verify_path: consider dos drive prefix

verify_path: consider dos drive prefix

If someone manage to create a repo with a 'C:' entry in the
root-tree, files can be written outside of the working-dir. This
opens up a can-of-worms of exploits.

Fix it by explicitly checking for a dos drive prefix when verifying
a paht. While we're at it, make sure that paths beginning with '\' is
considered absolute as well.

Noticed-by: Theo Niessink <redacted>
Signed-off-by: Erik Faye-Lund <redacted>
Signed-off-by: Junio C Hamano <redacted>