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
$ hg init
$ $PYTHON -c 'open("a", "wb").write(b"confuse str.splitlines\nembedded\rnewline\n")'
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\r (no-eol) (esc)
newline
+clean diff