git-p4: support exclude paths

git-p4: support exclude paths

Teach git-p4 about the -/ option which adds depot paths to the exclude
list, used when cloning. The option is chosen such that the natural
Perforce syntax works, eg:

  git p4 clone //branch/path/... -//branch/path/{large,old}/...

Trailing ... on exclude paths are optional.

This is a generalization of a change by Dmitry Kakurin (thanks).

Signed-off-by: Tommy Thorn <tommy-git@thorn.ws>
Signed-off-by: Simon Hausmann <simon@lst.de>