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.
#require rust
$ cat >> $HGRCPATH << EOF
> [format]
> use-dirstate-v2=1
> [storage]
> dirstate-v2.slow-path=allow
> EOF
$ hg init t
$ cd t
$ for i in 1 2 3 4 5 6 7 8 9 10; do touch foobar$i; done
$ hg add .
adding foobar1
adding foobar10
adding foobar2
adding foobar3
adding foobar4
adding foobar5
adding foobar6
adding foobar7
adding foobar8
adding foobar9
$ hg commit -m "1"
Check that there's no space leak on debugrebuilddirstate
$ f --size .hg/dirstate*
.hg/dirstate: size=133
.hg/dirstate.88698448: size=511
$ hg debugrebuilddirstate
$ f --size .hg/dirstate*
.hg/dirstate: size=133
.hg/dirstate.6b8ab34b: size=511
$ hg debugrebuilddirstate
$ f --size .hg/dirstate*
.hg/dirstate: size=133
.hg/dirstate.b875dfc5: size=511