Implement auto gc

Implement auto gc

With the auto option, gc checks whether any housekeeping is required; if
not, it exits without performing any work. Some JGit commands run gc
--auto after performing operations that could create many loose objects.
Housekeeping is required if there are too many loose objects or too many
packs in the repository.

If the number of loose objects exceeds the value of the gc.auto option
jgit's GC consolidates all existing packs into a single pack (equivalent
to -A option), whereas git-core would combine all loose objects into a
single pack using repack -d -l. Setting the value of gc.auto to 0
disables automatic packing of loose objects.

If the number of packs exceeds the value of gc.autoPackLimit, then
existing packs (except those marked with a .keep file) are consolidated
into a single pack by using the -A option of repack. Setting
gc.autoPackLimit to 0 disables automatic consolidation of packs.

Like git the following jgit commands run auto gc:
- fetch
- merge
- rebase
- receive-pack

The auto gc for receive-pack can be suppressed by setting the config
option receive.autogc = false

Change-Id: I68a2a051b39ec2c53cb7c4b8f6c596ba65eeba5d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>