[mural] Be smarter about picking the tagfile to use
[mural] Be smarter about picking the tagfile to use

Summary:
Update tagfile selection logic, in order of preference:
 1. tagfile associated with the current open buffer
 2. last used tagfile
 3. iff there's a single tagfile configured, use that
 else, error

Test Plan:
open new emacs with one tagfile configured, start in *scratch*.
open-dwim works.
open new emacs with two tagfiles configured, start in *scratch*,
open-dwim errors.
switch to a valid file, it now works.  switch back to *scratch*, works still




cloudrain21/mural,ahupp/mural