bug#21763: bug#22239: bug#22357: grep -f not only huge memory usage, but also huge time cost

[Paul Eggert]

Norihiro Tanaka wrote:
> Hmm, how about the following test cases, although it is extreame?

I don't think we need to worry about performance for the case when -w is given, 
and a pattern matches data that contains non-word characters. In practice, such 
cases are rare. I expect that most users would be surprised that -w can match 
non-word characters, and that users wouldn't object to -w rejecting such matches 
(if this wouldn't hurt performance significantly).

While looking into this I did find a very small performance tweak for the test 
case, and installed the attached.

0001-grep-minor-performance-tweak-for-pure-functions.patch
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