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grep: stop computing information for --diff when unnecessary

Description

grep: stop computing information for --diff when unnecessary

This is one reason why hg grep pattern essentially does `hg cat -r
. 'set:**'` inside. There is no speed improvement in this commit,
because the rest of the code still greps data from filelog instead of
working copy when possible.

Differential Revision: https://phab.mercurial-scm.org/D8544