Support "git pull" post hook alongside "git commit" pre hook

[vvambo]

I haven't looked deeply into git pre-hooks, but I imagine the ppl using this could provide instructions (or even upgrade the init.sh with an option) to make the git-meta commands run whenever an commit is done (before) and after a pull is done as to provide a seamless experience.

[danimesq]

@vvambo it already runs at a pre-commit hook (pls remember that it is installed manually per repo, by placing git-meta.sh and init.sh then running init.sh). Storing timestamps after a pull/merging is an amazing idea I haven't though about before

[frferrara]

As an addition, the post-checkout hook could also be adapted (such that the metadata is also updated when a branch is checked out).