view: Support annex.maxextensionlength when generating filenames for the
view branch.
Note that refining an existing view will reuse the extension length that was
configured when initially constructing the view. This is necessarily the case
because it reuses the filenames.
Also view files used to have all extensions at the end, no matter how
many there were. Since annex.maxextensionlength's documentation includes
that it's limited to 2 extensions, I made it consistent with that.
Sponsored-by: k0ld on Patreon
