Mercurial 5.1.1 released

[Augie Fackler]

Please update your packaged builds, thanks.

Release notes:

=== commands ===
 * bookmarks: actual fix for race condition deleting bookmark

=== core ===
 * exchange: abort on pushing bookmarks pointing to secret changesets (Bts:issue6159)
 * python-zstandard: apply big-endian fix (Bts:issue6188)
 * fncache: make debugrebuildfncache not fail on broken fncache

=== tests ===
 * various test runner fixes
 * fixed use of `tr --delete` for tr(1) portability