[BOLT][Utils] Pass cmp-rev to nfc-check-setup

[Amir]

Summary

Pass the revision to checkout to (cmp-rev) as nfc-check-setup option.
Simpifies the comparison against arbitrary commit, not just the previous one.

bolt/utils/nfc-check-setup.py

parser.add_argument(
         help="Path to BOLT build directory, default is current " "directory",
     )
     parser.add_argument(
         "--switch-back",
         default=False,
         action="store_true",
         help="Checkout back to the starting revision",
     )
+    parser.add_argument(
+        "--cmp-rev",
+        default="HEAD^",
+        help="Revision to checkout to compare vs HEAD",
+    )
     args, wrapper_args = parser.parse_known_args()
     bolt_path = f"{args.build_dir}/bin/llvm-bolt"
 
     source_dir = None
     # find the repo directory
     with open(f"{args.build_dir}/CMakeCache.txt") as f:
         for line in f:
             m = re.match(r"LLVM_SOURCE_DIR:STATIC=(.*)", line)
stash = subprocess.run(
         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 push -u"), cwd=source_dir)
-    # check out the previous commit
-    subprocess.run(shlex.split("git checkout -f HEAD^"), cwd=source_dir)
+    # check out the previous/cmp commit
+    subprocess.run(shlex.split(f"git checkout -f {args.cmp_rev}"), 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
     os.replace(bolt_path, f"{bolt_path}.old")