lint: Setup gitlint.

Setup gitlint for developers to write well formatted
commit messages.

Note: .gitlint, gitlint-rules.py and lint-commits
are taken directly from zulip/zulip with minor changes.

tools/lint-commits

@@ -0,0 +1,27 @@
+#!/bin/bash
+
+# This file is copied from the original tools/commit-message-lint at zulip/zulip,
+# Edited at Line 14 Col 97 (zulip -> python-zulip-api)
+# Please don't edit here; instead update the zulip/zulip copy and then resync this file.
+
+# Lint all commit messages that are newer than upstream/master if running
+# locally or the commits in the push or PR Gh-Actions.
+
+# The rules can be found in /.gitlint
+
+if [[ "
+$(git remote -v)
+" =~ '
+'([^[:space:]]*)[[:space:]]*(https://github\.com/|ssh://git@github\.com/|git@github\.com:)zulip/python-zulip-api(\.git|/)?\ \(fetch\)'
+' ]]; then
+    range="${BASH_REMATCH[1]}/master..HEAD"
+else
+    range="upstream/master..HEAD"
+fi
+
+commits=$(git log "$range" | wc -l)
+if [ "$commits" -gt 0 ]; then
+    # Only run gitlint with non-empty commit lists, to avoid a printed
+    # warning.
+    gitlint --commits "$range"
+fi