grep: hide cryptic revision number by default
I believe nobody would want to see the "2147483647:" prefix while grepping
working directory files.
The wdir revision is still visible if "-r wdir()" is specified since a revset
may point to more than one revisions, and it seems confusing to change the
visibility dynamically by the number of the matched revisions.
Differential Revision: https://phab.mercurial-scm.org/D3920