Re: [PATCH v5 4/4] submodule: record superproject gitdir during 'update'

Emily Shaffer [off-list ref] writes:

A recorded hint path to the superproject's gitdir might be added during
'git submodule add', but in some cases - like submodules which were
created before 'git submodule add' learned to record that info - it might
be useful to update the hint. Let's do it during 'git submodule
update', when we already have a handle to the superproject while calling
operations on the submodules.

We are hearing repeated mention of "cache" and "hint".  Do we ever
invalidate it, or if we have such a record, do we blindly trust it
and use it without verifying if it is still fresh?

Also, this step and the previous step both say we record gitdir on
their title, but we instead record common dir.  Whichever is the
right choice to record, let's be consistent.

Thanks.