Merge bitcoin/bitcoin#29660: lint: Fix COMMIT_RANGE issues

fa1146d01b lint: Fix COMMIT_RANGE issues (MarcoFalke)

Pull request description:

  `COMMIT_RANGE` has problems on forks or local branches:

  * When `LOCAL_BRANCH` is set, it assumes the presence of a `master` branch, and that the `master` branch is up-to-date. Both of which may be false. (See also discussion in https://github.com/bitcoin/bitcoin/pull/29274#discussion_r1504226422)
  * When `COMMIT_RANGE` isn't set in `lint-git-commit-check.py`, and `--prev-commits` isn't set either, it has the same (broken) assumptions.

  Fix all issues by simply assuming a merge commit exists. This allows to drop `LOCAL_BRANCH`. It also allows to drop `SKIP_EMPTY_NOT_A_PR`, because scripts will already skip an empty range. Finally, it allows to drop `--prev-commits n`, because one can simply say `COMMIT_RANGE='HEAD~n..HEAD'` to achieve the same.

ACKs for top commit:
  Sjors:
    tACK fa1146d01b

Tree-SHA512: f1477a38267fd4fdb8d396211a5d6bed5f418798c7edaba43487957aaf726cd45244ccf15187b3dd896d398fa1df3fe0a37323e49cf44d60a2018786ed41e5ba

test/lint/lint-git-commit-check.py

@@ -23,31 +23,18 @@ def parse_args():
         """,
         epilog=f"""
             You can manually set the commit-range with the COMMIT_RANGE
-            environment variable (e.g. "COMMIT_RANGE='47ba2c3...ee50c9e'
-            {sys.argv[0]}"). Defaults to current merge base when neither
-            prev-commits nor the environment variable is set.
+            environment variable (e.g. "COMMIT_RANGE='HEAD~n..HEAD'
+            {sys.argv[0]}") for the last 'n' commits.
         """)
-
-    parser.add_argument("--prev-commits", "-p", required=False, help="The previous n commits to check")
-
     return parser.parse_args()
 
 
 def main():
-    args = parse_args()
+    parse_args()
     exit_code = 0
 
-    if not os.getenv("COMMIT_RANGE"):
-        if args.prev_commits:
-            commit_range = "HEAD~" + args.prev_commits + "...HEAD"
-        else:
-            # This assumes that the target branch of the pull request will be master.
-            merge_base = check_output(["git", "merge-base", "HEAD", "master"], text=True, encoding="utf8").rstrip("\n")
-            commit_range = merge_base + "..HEAD"
-    else:
-        commit_range = os.getenv("COMMIT_RANGE")
-        if commit_range == "SKIP_EMPTY_NOT_A_PR":
-            sys.exit(0)
+    assert os.getenv("COMMIT_RANGE")  # E.g. COMMIT_RANGE='HEAD~n..HEAD'
+    commit_range = os.getenv("COMMIT_RANGE")
 
     commit_hashes = check_output(["git", "log", commit_range, "--format=%H"], text=True, encoding="utf8").splitlines()