hooks: add a post-commit hook

hooks: add a post-commit hook

Under normal Girocco operations a post-commit hook will never need
to be run under any circumstances for any normal Girocco operations.

However, if linked working trees are in use or even, gasp, the
Girocco repository isn't actually bare, commits could be taking
place outside Girocco's control.

If the $Girocco::Config::autogchack has been activated, these will
eventually get noticed automatically at some point.

However, the most common way commits would be added and escape the
$autogchack detection mechanism would be via the simple "git commit"
command.

Provide a post-commit hook to catch this and run the same $autogchack
logic if that has been enabled.  As a nice bonus it will also cause
a "mini" gc to be requested should one be needed.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>