dirstate: fix compilation warnings in dirstate_item_set_possibly_dirty()
Since https://phab.mercurial-scm.org/D11387 (i.e. the same patch as
mentioned in my previous patch), Clang has also started warning about
dirstate_item_set_possibly_dirty() missing an explicit return, and
about its use of the result of an assignment as a condition without
using parentheses. This patch fixes that.
Differential Revision: https://phab.mercurial-scm.org/D11445