Bug 1669332 - Add specific error message singling out gecko-dev if no hg revisions for the git repo can be found r=firefox-build-system-reviewers,mhentges,glandium

People keep using `gecko-dev` and trying to run artifact builds, although this very unsupported, and the existing error message is useless if you're not knowledgeable about how these systems work. Since this is the most common case where people come in with questions about artifact builds not working, try to detect this case and print a helpful error message.

Differential Revision: https://phabricator.services.mozilla.com/D92492

python/mozbuild/mozbuild/artifacts.py

@@ -994,12 +994,25 @@ class Artifacts(object):
                 continue
             hashes.append(hg_hash)
         if not hashes:
-            raise UserError(
-                'Could not list any recent revisions in your clone. Does your '
-                'clone have git-cinnabar metadata? If not, consider re-cloning '
-                'using the directions at '
-                'https://github.com/glandium/git-cinnabar/wiki/Mozilla:-A-git-'
-                'workflow-for-Gecko-development')
+            msg = ('Could not list any recent revisions in your clone. Does '
+                   'your clone have git-cinnabar metadata? If not, consider '
+                   're-cloning using the directions at '
+                   'https://github.com/glandium/git-cinnabar/wiki/Mozilla:-A-'
+                   'git-workflow-for-Gecko-development')
+            try:
+                subprocess.check_output(
+                    [self._git, 'cat-file', '-e',
+                     '05e5d33a570d48aed58b2d38f5dfc0a7870ff8d3^{commit}'],
+                    stderr=subprocess.STDOUT)
+                # If the above commit exists, we're probably in a clone of
+                # `gecko-dev`, and this documentation applies.
+                msg += ('\n\nNOTE: Consider following the directions '
+                        'at https://github.com/glandium/git-cinnabar/wiki/'
+                        'Mozilla:-Using-a-git-clone-of-gecko%E2%80%90dev-'
+                        'to-push-to-mercurial to resolve this issue.')
+            except subprocess.CalledProcessError:
+                pass
+            raise UserError(msg)
         return hashes
 
     def _get_recent_public_revisions(self):