merge: fix spurious merges for copies in linear updates We make better use of contexts to accurately identify copies that don't need merges. Add a simple test and update other tests.

branch.cache: silently ignore I/O and OS errors

context: fix workingfilectx._changectx

mercurial.el: don't raise an error from find-file-hook find-file() shouldn't fail even if unreadable .hg directory is exists. Not enabling hg-mode is sufficient for that senario.