Allow snapshot of subdirectories

Gitweb is a fantastic tool to colaborate with colleagues (managers, ...) not willing or able to use Git itself but being able to utilize a browser. Unforunately Gitweb snapshot functionality didn't allow to pull snaphots of subdirectories which can result in way too big snapshots containing unwanted information. This little change allows to call URLs like "<gitweburl>?p=<repo>;a=snapshot;f=<subdir>;h=HEAD" and enables users to specify scopes more exact.