Use blobless git clone (--filter=blob:none --no-checkout) to fetch
commit history for content from Git modules, then map it using the
existing gitmap library. Cloned repos are cached in the modulegitinfo file cache.
Closes#14431Fixes#5533
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
Bjørn Erik Pedersen