tests: handle deleted .hg directory (git 2.2.0 and higher) (issue4585)

In git 2.2.0 and higher, removing files and directories is changed:
removing an object that does not exist returns success rather than failure.
As a result, even though .hg/hgrc does not exist, success is returned
and the .hg/ directory is removed.

To handle this correctly, use 'rm -rf' to allow successful removing
for all git versions.

The exact changeset where this was introduced in git:
1054af7d04aef64378d69a0496b45cdbf6a0bef2
wrapper.c: remove/unlink_or_warn: simplify, treat ENOENT as success

tests/test-subrepo-git.t

@@ -1002,7 +1002,7 b' since we ignore the staging area'
   $ cd s
   $ rm snake.python
 (remove leftover .hg so Mercurial doesn't look for a root here)
-  $ rm -r .hg
+  $ rm -rf .hg
   $ hg status --subrepos --all .
   R snake.python
   ? barfoo