Move file modification to after branch checkout

Change-Id: I62116db5a7181526f713850d42704c8e74fe8741
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363330Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/master@{#799214}

tools/clang/scripts/upload_revision.py

@@ -96,10 +96,11 @@ def main():
   print("Making a patch for Clang {}-{}".format(clang_git_revision,
                                                 clang_sub_revision))
 
-  old_rev_string = PatchRevision(clang_git_revision, clang_sub_revision)
-
   rev_string = "{}-{}".format(clang_git_revision, clang_sub_revision)
   Git(["checkout", "origin/master", "-b", "clang-{}".format(rev_string)])
+
+  old_rev_string = PatchRevision(clang_git_revision, clang_sub_revision)
+
   Git(["add", UPDATE_PY_PATH])
 
   commit_message = 'Ran `{}`.'.format(' '.join(sys.argv)) + COMMIT_FOOTER