Add filtering by file path in 'git-pw list'

Vaguely connected to https://github.com/dlespiau/patchwork/issues/139 but I didn't want to hijack that issue.

With pwclient I do (for example) 'pwclient list msm' to list just patches w/ msm in the subject. It would be nice to be able to do similar. Although perhaps even better would be filtering on path (so you could just list the driver path(s) that you are interested in and see all the patches that touch the driver/subsystem/etc that you maintain.