wimlib-imagex: try harder to optimize out opening template WIM
As an optimization, 'wimcapture' and 'wimappend' don't separately open
the template WIM for --update-of if no filename is specified in that
option, which makes it default to either the single base WIM
(--delta-from), or the WIM being appended to.
Extend that optimization to cases where the filename is specified in
--update-of and it exactly matches the filename of the WIM being
appended to or any of the base WIMs.
git://wimlib.net / wimlib / commit