amend: rectify comment

Comment was ambiguous as there can be two parents of a changeset in mercurial.
This commit fixes the comment to clarify that the first parent is being
considered.

Test Plan:
ran the test suite

Differential Revision: https://phab.mercurial-scm.org/D595

mercurial/cmdutil.py

@@ -3069,7 +3069,7 b' def amend(ui, repo, commitfunc, old, ext'
         #          |
         # old      o - changeset to amend
         #          |
-        # base     o - parent of amending changeset
+        # base     o - first parent of the changeset to amend
 
         # Update extra dict from amended commit (e.g. to preserve graft
         # source)