Add process/file lock to gvfs-helper prefetch

[jeffhostetler]

As an optimization we could add a file lock to gvfs-helper prefetch to make
concurrent prefetches operate serially.

Ideally, multiple prefetches on a single repo would be hitting the same cache-server
(or origin server) and by default only ask for packs newer than the most recent one
in the shared-cache. So both processes would probably always receive the same
set of pack and waste network bandwidth, since the loser of the race will just discard
their copy of the packs.

A file lock could be added to avoid this.

This is probably only important for the highly-concurrent GVFS test suite.
Normal users are probably unlikely to run concurrent git fetch commands.