localrepo: add sparse caches
The sparse extension maintains caches for the sparse files
to a signature and a signature to a matcher. This allows the
sparse matchers to be resolved quickly, which is apparently
something that can occur in loops.
This patch ports the sparse caches to the localrepo class
pretty much as-is. There is potentially room to improve the
caching mechanism. But that can be done as a follow-up.
The default invalidatecaches() now clears the relevant sparse
cache. invalidatesignaturecache() has been moved to sparse.py.
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors