perf: use the `perf--` prefix for perf command
This is the one command namespace where they should not be any ambiguity about
command that should be in it. The perf extensions is only adding performance
related command.
so this is a good ground to start putting dash folding to the tests.
Differential Revision: https://phab.mercurial-scm.org/D9516
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1