perf-stream-locked-section: advertise the right version key in the help
As the v3 format is still experimental, its key is "v3-exp". The help text was
not pointing that out.
(we also fix `perf::stream-generate` in the process)
$ 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