gc: notice gc processes run by other users

gc: notice gc processes run by other users

Since 64a99eb4 git gc refuses to run without the --force option if
another gc process on the same repository is already running.

However, if the repository is shared and user A runs git gc on the
repository and while that gc is still running user B runs git gc on
the same repository the gc process run by user A will not be noticed
and the gc run by user B will go ahead and run.

The problem is that the kill(pid, 0) test fails with an EPERM error
since user B is not allowed to signal processes owned by user A
(unless user B is root).

Update the test to recognize an EPERM error as meaning the process
exists and another gc should not be run (unless --force is given).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>