revlog: optionally cache the full text when adding revisions revlog instances can cache the full text of a single revision. Typically the most recently read revision is cached. When adding a delta group via addgroup() and _addrevision(), the full text isn't always computed: sometimes only the passed in delta is sufficient for adding a new revision to the revlog. When writing the changelog from a delta group, the just-added full text revision is always read immediately after it is written because the changegroup code needs to extract the set of files from the entry. In other words, revision() is *always* being called and caching the full text of the just-added revision is guaranteed to result in a cache hit, making the cache worthwhile. This patch adds support to _addrevision() for always building and caching the full text. This option is currently only active when processing changelog entries from a changegroup. While the total number of revision() calls is the same, the location matters: buildtext() calls into revision() on the base revision when building the full text of the just-added revision. Since the previous revision's _addrevision() built the full text and the the previous revision is likely the base revision, this means that the base revision's full text is likely cached and can be used to compute the current full text from just a delta. No extra I/O required. The end result is the changelog isn't opened and read after adding every revision from a changegroup. On my 2013 MacBook Pro running OS X 10.10.5 from an SSD and Python 2.7, this patch impacted the time taken to apply ~262,000 changesets from a mozilla-central gzip bundle: before: ~43s after: ~32s ~25% reduction in changelog processing times. Not bad.