context: make warning message for hidden revision extensible
Extensions might want to create new filternames and change what revisions
are considered hidden or shown. This is the case for inhibit that enables
direct access to hidden hashes with the visible-directaccess-nowarn filtername.
By using startswith instead of a direct comparison with 'visible' we
allow extensions to do that and not work directly on the 'visible' filtername
used by core.
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