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.
Create an empty repo:
$ hg init a
$ cd a
Try some commands:
$ hg log
$ hg grep wah
[1]
$ hg manifest
$ hg verify -q
Check the basic files created:
$ ls .hg
00changelog.i
cache
requires
store
wcache
Should be empty (except for the "basic" requires):
$ ls .hg/store
requires
Poke at a clone:
$ cd ..
$ hg clone a b
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd b
$ hg verify -q
$ ls .hg
00changelog.i
branch
cache
hgrc
requires
store
wcache
Should be empty (except for the "basic" requires):
$ ls .hg/store
requires
$ cd ..