Add tests for handling pack files removal during fetch

Add tests for handling pack files removal during fetch

Although this could sound like a corner case, it really can occur out
there in the real world. Especially in the Gerrit world where the
repositories could be GC'ed on a separate process or system.

The `FileNotFoundException` seems to be handled correctly in
`PackFile#doOpen` (line 671) and it will mark the pack as invalid. But
triggering that code path was not an easy task.

First of all, we need to add a new commit to the `master` branch of the
test repository after `UploadPack` object is created.

Secondly, in the refspec for fetch, commit id instead of "regular"
refspec must be used.

With both in place, we can see a warning log statement about deleted
pack file. And the fetch succeeds!

Also, tests for the removal of *.idx and *.bitmap files were added.

This unveiled a corner for the *.idx file deletion while fetching, as
the test will fail with "Unreachable pack index" IOException only
when the HEAD commit is empty.

Change-Id: If26c83f9b12993d1ab7d6bad6bd863c29520b062
Signed-off-by: Dariusz Luksza <dariusz.luksza@gmail.com>