The callers of git_packfile_unpack() expect the obj_offset argument to
be set to the beginning of the next object. We were mistakenly returning
the the offset of the object's data, which causes the CRC function to
try to use the wrong offset.

Set obj_offset to curpos instead of elem->offset to point to the next
element and bring back expected behaviour.

If we fail to insert the packfile in the map, make sure to free it.

This makes the free function only attempt to remove its mwindows from
the global list if we have opened the packfile to avoid accessing the
list unlocked.

Opening the same repository multiple times will currently open the same
file multiple times, as well as map the same region of the file multiple
times. This is not necessary, as the packfile data is immutable.

Instead of opening and closing packfiles directly, introduce an
indirection and allocate packfiles globally. This does mean locking on
each packfile open, but we already use this lock for the global mwindow
list so it doesn't introduce a new contention point.

When running multithreaded, it is not enough to check for the offmap
allocation. Move the call to cache_init() to packfile allocation so we
can be sure it is always allocated free of races.

This fixes #2355.

The base object is a good cache candidate, so we shouldn't forget to add
it to the cache.