Detect work-in-progress commits in danger CI

To avoid accidentally merging unfinished work, detect prohibited
keywords at the start of the subject line. If the first word is any of
the following, fail the check:
WIP, wip, DROP, drop, TODO, todo

The only slightly controversial is the lowercase "drop" which might have
a legitimate use - seems like four commits in the history used it as a
start of a sentence. However, since people commonly use "drop" to
indicate a commit should be dropped before merging, let's prohibit it as
well. In case of false-positive, "Drop" with a capitalized first letter
can always be used.

(cherry picked from commit 402b11431c)

dangerfile.py

@@ -62,6 +62,9 @@ mr = proj.mergerequests.get(os.environ["CI_MERGE_REQUEST_IID"])
 #
 #     * The subject line starts with "fixup!" or "Apply suggestion".
 #
+#     * The subject line starts with a prohibited word indicating a work in
+#       progress commit (e.g. "WIP").
+#
 #     * The subject line contains a trailing dot.
 #
 #     * There is no empty line between the subject line and the log message.
@@ -87,6 +90,9 @@ mr = proj.mergerequests.get(os.environ["CI_MERGE_REQUEST_IID"])
 #           "[2]", etc.) which allows e.g. long URLs to be included in the
 #           commit log message.
 
+PROHIBITED_WORDS_RE = re.compile(
+    "^(WIP|wip|DROP|drop|DROPME|checkpoint|experiment|TODO|todo)[^a-zA-Z]"
+)
 fixup_error_logged = False
 for commit in danger.git.commits:
     message_lines = commit.message.splitlines()
@@ -99,6 +105,12 @@ for commit in danger.git.commits:
             "Please squash them before merging."
         )
         fixup_error_logged = True
+    match = PROHIBITED_WORDS_RE.search(subject)
+    if match:
+        fail(
+            f"Prohibited keyword `{match.groups()[0]}` detected "
+            f"at the start of a subject line in commit {commit.sha}."
+        )
     if len(subject) > 72 and not subject.startswith("Merge branch "):
         warn(
             f"Subject line for commit {commit.sha} is too long: "