perldoc diffs: don't search auto - much faster

perldoc diffs: don't search auto - much faster

Give this a try.  It causes unsuccessful searches to run about
3 times as fast on my machine.  Not exactly a speed demon but
at least you don't have time to get up and make coffee.

I tried adding caching to stop the repeated readdir in the
case-insensitive matching subroutine, but a) it only lopped off
another 20% and b) the patch made a kind of a Frankenstein thing
out of the subroutine, so c) I will try to write a better one
later when I have time.

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