configs annex.post-update-command and annex.pre-commit-command

Added git configs annex.post-update-command and annex.pre-commit-command
that correspond to the git-annex hook scripts post-update-annex and
pre-commit-annex.

Note that the hook files take precience over the git config, since the git
config can includ global config which should be overridden by local config.

These new git configs are probably not super useful. Especially the
pre-commit-annex hook is there to install scripts to instead of the
pre-commit hook, since git-annex installs that hook itself. So why would
someone want to use a git config for that? Only reason I can think of would
be in a global git config. Or possibly because it's easier to set a git
config than write a hook script, on an OS like Windows.

The real reason I'm adding these is as groundwork for making other
annex.*-command git configs also be available as hook scripts. I want
to avoid having some things available as only git hooks and others as
both gitconfigs and git hooks. (It seems that some annex.*-command configs
don't translate to git hooks though.)

In the man page, moved documentation of the hooks to be next to the
documentation of the git configs. This is to avoid repitition.

Command/PreCommit.hs

@@ -44,7 +44,7 @@ seek ps = do
 	-- files in the worktree won't be populated, so populate them here
 	Command.Smudge.updateSmudged (Restage False)
 	
-	runAnnexHook preCommitAnnexHook
+	runAnnexHook preCommitAnnexHook annexPreCommitCommand
 
 	-- committing changes to a view updates metadata
 	currentView >>= \case