If the private index file matches a previously known revision of the
database, we can update the index incrementally using the recorded
lastmod counter. This is typically much faster than a full update,
although it could be slower in the case of large changes to the
database.
The "git-read-tree HEAD" is also a bottleneck, but unfortunately
sometimes is needed. Cache the index checksum and hash to reduce the
number of times the operation is run. The overall design is a
simplified version of the PrivateIndex class.
Unlike the (current) infix query parser provided by Xapian, the
notmuch specific sexp query parser supports prefixed wildcard queries,
so use those. In addition to being somewhat faster, this avoids
needing to escape all of the user's tags to pass via the shell.
Although the code required to support both new and old environment
variables is small, it complicates the semantics of configuration, and
make the documentation harder to follow.
