v0.54.0

Changed — every .gitignore of a source counts

Only the root .gitignore was read. A tool that keeps its own .gitignore
in its directory — a ticket tracker beside its database, a build cache —
had its internals indexed anyway: on one project, files rewritten at every
ticket operation were re-indexed hundreds of times a day. Each .gitignore
now applies below its directory, a deeper one overriding a shallower one,
and nothing is brought back from inside an ignored directory, as in git.
The scan and the change watcher no longer enter a directory a rule leaves
out. A .gitignore that changes anywhere in the tree triggers a scan.

Fixed — a file a new ignore rule leaves out leaves the index

A file indexed before a rule appeared stayed in the index for good: the
scan stopped offering it, and the removal was held back because the file
was still on disk. A build cache that git was told to ignore kept 1,678
files searchable that way. A file an ignore rule now leaves out is taken
out at the next scan, present or not. The removal checks the rule again
before acting, so a rule taken back in between keeps the file; an ignore
file that cannot be read adds no rule, so a mount gone dark removes nothing.