Rollup merge of #148500 - Kobzol:git-update-inex, r=jieyouxu

Update git index before running diff-index

Discussed in https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/tidy.3A.20strange.20number.20of.20modified.20files/with/553714742.

This is apparently the cause of `x test tidy` printing weird number of formatted files, and also of sometimes quirky behavior of finding the files modified from a base commit.

Author: Zalathar