revset: use nullrev constant in merge()

mixedrepostorecache: fix a silly redundant updating of set Differential Revision: https://phab.mercurial-scm.org/D6470

rust-regex: fix shortcut for exact matches The current shortcut for rootglobs that can be simplified to exact matches does not work, it instead treats the pattern as a regex, which is not the same thing. This changes fixes the behavior and introduces a test for this behavior. Differential Revision: https://phab.mercurial-scm.org/D6489