Use the timestamp of .git/FETCH_HEAD in each git repository, to
determine if too much time has passed since the last fetch/pull.
Modify pmb.helpers.git.clone, so FETCH_HEAD is always created if it does
not exist (because "git clone" would not create it).
Related: #1829
Extend "pmbootstrap status" with checks for all git repositories,
derived from relevant checks in pmb.helpers.git.pull (using shared
code):
* on official branch
* workdir is clean
* tracking proper remote
* up to date
Related: #1829
Add new "pmbootstrap status" command, which does a quick health check
for the work dir. As first health check, verify that the chroots are not
too old. Replace the reminder text at the end of "pmbootstrap init" to
tell users to run "pmbootstrap status" instead of "pmbootstrap zap" once
a day before working with pmbootstrap.
Related: #1829
