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>

builtin/gc.c

@@ -222,7 +222,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 			time(NULL) - st.st_mtime <= 12 * 3600 &&
 			fscanf(fp, "%"PRIuMAX" %127c", &pid, locking_host) == 2 &&
 			/* be gentle to concurrent "gc" on remote hosts */
-			(strcmp(locking_host, my_host) || !kill(pid, 0));
+			(strcmp(locking_host, my_host) || !kill(pid, 0) || errno == EPERM);
 		if (fp != NULL)
 			fclose(fp);
 		if (should_exit) {