Create 'git gc' to perform common maintenance operations.

Create 'git gc' to perform common maintenance operations.

Junio asked for a 'git gc' utility which users can execute on a
regular basis to perform basic repository actions such as:

 * pack-refs --prune
 * reflog expire
 * repack -a -d
 * prune
 * rerere gc

So here is a command which does exactly that.  The parameters fed
to reflog's expire subcommand can be chosen by the user by setting
configuration options in .git/config (or ~/.gitconfig), as users may
want different expiration windows for each repository but shouldn't
be bothered to remember what they are all of the time.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>