Allow to discover bitmap on disk created after the packfile

Allow to discover bitmap on disk created after the packfile

When the bitmap file was created *after* a packfile had been
loaded into the memory, JGit was unable to discover them.

That happed because of two problems:

1. The PackDirectory.getPacks() does not implement the usual
   while loop that is scanning through the packs directory
   as in the other parts of JGit.

2. The scan packs does not look for newly created bitmap files
   if the packfile is already loaded in memory.

Implement the normal packfiles scanning whenever the PackDirectory
needs to return a list of packs, and make sure that any reused
Pack object would have its associated bitmap properly refreshed
from disk.

Adapt the assertions in GcConcurrentTest with the rescanned list
of Pack from the objects/packs directory.

Bug: jgit-15
Change-Id: I2ed576cefd78a0e128b175228a59c9af51523d7b