lazily close revlog files and keep up to 8
revlogs arround. also cache the latest extracted
file descriptor of a revision in the revlog.
this avoids the quite expensive reextracting/patching
when we reopen the same file revision.
dont use the racy mktemp()/create, instead create
a uniqueue name and create with OEXCL. this also
avoids a bunch of access() calls.
fix eof case and use pread() in fcopy() to avoid the
seeks.
dont modify changelog temp file but simulate trailing
newline instead.
