branchcache: skip entries that are topological heads in the on disk file
In the majority of cases, topological heads are also branch heads. We have
efficient way to get the topological heads and efficient way to retrieve
their branch information. So there is little value in putting them in the branch
cache file explicitly. On the contrary, writing them explicitly tend to create
very large cache file that are inefficient to read and update.
So the branch cache v3 format is no longer including them. This changeset focus
on the format aspect and have no focus on the performance aspect. We will cover
that later.
#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