git-svn gc: acquire a `gc.pid` lock for any packing activities

git-svn gc: acquire a `gc.pid` lock for any packing activities

It really should not be possible for any kind of gc to run on
an svn mirror project while a clone/update operation is in progress.

It really shouldn't.

Nevertheless, do the right thing and acquire a `gc.pic` lock
for the duration of any packing operations.

Since, strictly speaking, no packing is actually required during
a clone or update of a `git-svn` mirror, failure to acquire
the lock only results in the packing operation being skipped
rather than an outright failure.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>