compat: add support for upstream rename of merge.update() to _update()

The function was renamed in Mercurial commit 2c86b9587740.
8 jobs for topic/default/compat-merge-update in 4 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed checks-py2 #108019

00:01:15

passed checks-py3 #108020

00:01:12

passed doc #108025

00:00:39

manual pytype #108018
allowed to fail manual
passed tests-py2-cext #108021

00:04:02

passed tests-py2-pure #108022

00:03:53

passed tests-py3-cext #108023

00:03:42

passed tests-py3-pure #108024

00:03:25