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

[Amir]

Summary

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.

[rafauler]

LGTM

bolt/utils/nfc-check-setup.py

def main():
     old_ref = get_git_ref_or_rev(source_dir)
 
     # determine whether a stash is needed
     stash = subprocess.run(shlex.split("git status --porcelain"), cwd=source_dir,
                            stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
                            text=True).stdout
     if stash:
         # save local changes before checkout
-        subprocess.run(shlex.split("git stash"), cwd=source_dir)
+        subprocess.run(shlex.split("git stash push -u"), cwd=source_dir)
     # check out the previous commit
     subprocess.run(shlex.split("git checkout -f HEAD^"), cwd=source_dir)
     # get the parent commit hash for logging
     new_ref = get_git_ref_or_rev(source_dir)
     # build the previous commit
     subprocess.run(shlex.split("cmake --build . --target llvm-bolt"),
                    cwd=args.build_dir)
     # rename llvm-bolt