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.
#testcases dirstate-v1 dirstate-v2
#if dirstate-v2
$ cat >> $HGRCPATH << EOF
> [format]
> use-dirstate-v2=1
> [storage]
> dirstate-v2.slow-path=allow
> EOF
#endif
$ rm -rf r
$ hg init r
$ cd r
$ mkdir d1
$ mkdir d2
$ touch d1/f d2/f
$ hg commit -Am '.'
adding d1/f
adding d2/f
$ echo 'syntax:re' >> .hgignore
$ echo '^d1$' >> .hgignore
$ hg commit -Am "ignore d1"
adding .hgignore
Now d1 is a directory that's both committed and ignored.
Untracked files in d2 are still shown, but ones in d1 are ignored:
$ touch d1/g
$ touch d2/g
$ RAYON_NUM_THREADS=1 hg status
? d2/g