revlog: use LRU for the chain cache
For a large repository, this reduces the number of filelog instances and
associated data a lot. For a 1% speed penalty, it reduces peak RSS by
20% for the full NetBSD test repository.
Differential Revision: https://phab.mercurial-scm.org/D9235
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: