amend: abort early if no username is configured with evolve enabled (issue4211)

Amend will reuse the original username if a new one is not provided
with -U, but obsolete.createmarkers() only considers ui.username() for
the obsolete marker's metadata. Allowing the metadata field to be
spoofed seems to defeat the point of the field in the first place.
This covers 'evolve amend' and 'ci --amend' with evolve enabled.

Without this, the transaction aborts but the parent changeset is set to -1.  The
corresponding test will be added to evolve separately.

mercurial/cmdutil.py

@@ -2121,6 +2121,11 b' def commit(ui, repo, commitfunc, pats, o'
                       scmutil.match(repo[None], pats, opts), opts)
 
 def amend(ui, repo, commitfunc, old, extra, pats, opts):
+    # amend will reuse the existing user if not specified, but the obsolete
+    # marker creation requires that the current user's name is specified.
+    if obsolete._enabled:
+        ui.username() # raise exception if username not set
+
     ui.note(_('amending changeset %s\n') % old)
     base = old.p1()