[BOLT][UTILS] Stash including untracked in nfc-check-setup

The command to detect whether the stash is needed is `git status --porcelain`
which includes untracked files by default. We want to stash untracked files
as well as they may affect compilation (LLVM CMake checks that all source files
should be included in CMakeLists).

Update the stash command to include untracked files as well.

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D132610