histedit: use hg.updaterepo() to avoid ui.{push|pop}buffer() hack
In some parts of code, we call hg.update() and surround that with
ui.pushbuffer() and ui.popbuffer() to suppress the output returned by
hg.update().
We have hg.updaterepo() which does not writes to UI and can be used instead.
Differential Revision: https://phab.mercurial-scm.org/D3862
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL