Rollup merge of #144511 - lolbinarycat:tidy-extra-checks-opt, r=Kobzol

tidy: increase performance of auto extra checks feature

Removes the repeated calls to git diff.

Halves the overhead of the tidy extra checks feature from 0.1 seconds to 0.05 on my machine, but probably will be more significant on systems on slow disks or less memory for i/o cache.

r? ``@Kobzol``

Author: Zalathar