branchcache-v3: use more explicit header line
The key-value approach is clearer and gives more rooms to have the format evolve
in a clear way. It also provides extension (like topic) simpler way to extend
the validation scheme.
This is just a small evolution, the V3 format is still a work in progress.
$ hg init repo
$ cd repo
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a