revert(commitlint): stop ignoring "fixup" commits (#29184)

This reverts 2875d45e79.

Allowing lintcommit to ignore "fixup" makes it too easy to fixup commits
to be merged on master as the CI won't give any indications that
something is wrong. Contributors can always squash their pull requests
if it annoys them too much.

scripts/lintcommit.lua

@@ -41,12 +41,6 @@ end
 -- Returns nil if the given commit message is valid, or returns a string
 -- message explaining why it is invalid.
 local function validate_commit(commit_message)
-  -- Return nil if the commit message starts with "fixup" as it signifies it's
-  -- a work in progress and shouldn't be linted yet.
-  if vim.startswith(commit_message, 'fixup') then
-    return nil
-  end
-
   local commit_split = vim.split(commit_message, ':', { plain = true })
   -- Return nil if the type is vim-patch since most of the normal rules don't
   -- apply.
@@ -229,9 +223,9 @@ function M._test()
     ['vim-patch:8.2.3374: Pyret files are not recognized (#15642)'] = true,
     ['vim-patch:8.1.1195,8.2.{3417,3419}'] = true,
     ['revert: "ci: use continue-on-error instead of "|| true""'] = true,
-    ['fixup'] = true,
-    ['fixup: commit message'] = true,
-    ['fixup! commit message'] = true,
+    ['fixup'] = false,
+    ['fixup: commit message'] = false,
+    ['fixup! commit message'] = false,
     [':no type before colon 1'] = false,
     [' :no type before colon 2'] = false,
     ['  :no type before colon 3'] = false,