largefiles: avoid printing messages while rebasing by "_lfstatuswriters"
Putting "lambda *msg, **opts: None" (= avoid printing messages always)
into "_lfstatuswriters" while rebasing makes explicit passing
"printmessage = False" for "updatelfiles()" useless.
This patch also removes setting/unsetting "repo._isrebasing" in
"overriderebase", because there is no code path referring it.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.