branchcache: add a "pure topological head" fast path
In a narrow but actually quick common case, all topological heads are all on
the same branch and all open. In this case, computing the branch map is very
simple. We can quickly detect situation where this situation will not change.
So we update the V3 format to be able to express this situation and upgrade the
update code to detect we remains in that mode.
The branch cache is populated with the actual value when the branch map is
accessed, but the update_disk method can do the update without needing to
populate it.
Tests about metadataonlyctx
$ hg init
$ echo A > A
$ hg commit -A A -m 'Add A'
$ echo B > B
$ hg commit -A B -m 'Add B'
$ hg rm A
$ echo C > C
$ echo B2 > B
$ hg add C -q
$ hg commit -m 'Remove A'
$ cat > metaedit.py <<EOF
> from mercurial import context, pycompat, registrar
> cmdtable = {}
> command = registrar.command(cmdtable)
> @command(b'metaedit')
> def metaedit(ui, repo, arg):
> # Modify commit message to "FOO"
> with repo.wlock(), repo.lock(), repo.transaction(b'metaedit'):
> old = repo[b'.']
> kwargs = dict(s.split(b'=', 1) for s in arg.split(b';'))
> if b'parents' in kwargs:
> kwargs[b'parents'] = map(int, kwargs[b'parents'].split(b','))
> new = context.metadataonlyctx(repo, old,
> **pycompat.strkwargs(kwargs))
> new.commit()
> EOF
$ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'text=Changed'
$ hg log -r tip
changeset: 3:ad83e9e00ec9
tag: tip
parent: 1:3afb7afe6632
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Changed
$ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'parents=0' 2>&1 | grep -E '^RuntimeError'
RuntimeError: can't reuse the manifest: its p1 doesn't match the new ctx p1
$ hg --config extensions.metaedit=$TESTTMP/metaedit.py metaedit 'user=foo <foo@example.com>'
$ hg log -r tip
changeset: 4:1f86eaeca92b
tag: tip
parent: 1:3afb7afe6632
user: foo <foo@example.com>
date: Thu Jan 01 00:00:00 1970 +0000
summary: Remove A