rebase: also restore "ui.allowemptycommit" value
It looks like this was lost when the code was converted to the
ui.configoverride() context manager in
f255b1811f5e (rebase: get rid
of ui.backupconfig, 2017-03-16). (And then the bad example was
duplicated in
228916ca12b5 (rebase: add concludememorynode(), and call
it when rebasing in-memory, 2017-12-07).)
Differential Revision: https://phab.mercurial-scm.org/D2760
[workspace]
members = ["hgcli"]