mergestate: add accessors for local and other nodeid, not just contexts
The mergestate can contain invalid nodeids. In that case,
`mergestate.localctx` or `mergestate.otherctx` will fail. This patch
provides a way of accessing the nodeid without failing in such cases.
Differential Revision: https://phab.mercurial-scm.org/D8040
$ cat >> $HGRCPATH << EOF
> [ui]
> logtemplate="{rev}:{node|short} ({phase}) [{tags} {bookmarks}] {desc|firstline}\n"
> [extensions]
> dirstateparanoidcheck = $TESTDIR/../contrib/dirstatenonnormalcheck.py
> [experimental]
> nonnormalparanoidcheck = True
> [devel]
> all-warnings=True
> EOF
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init testrepo
$ cd testrepo
$ mkcommit a
$ mkcommit b
$ mkcommit c
$ hg status