histedit: support editing of the first commit (issue3767)

hgext/histedit.py

@@ -580,6 +580,7 b' def bootstrapcontinue(ui, repo, parentct'
     # note: does not take non linear new change in account (but previous
     #       implementation didn't used them anyway (issue3655)
     newchildren = [c.node() for c in repo.set('(%d::.)', parentctx)]
+    if parentctx.node() != node.nullid:
     if not newchildren:
         # `parentctxnode` should match but no result. This means that
         # currentnode is not a descendant from parentctxnode.

tests/test-histedit-edit.t

@@ -209,3 +209,16 b' rollback should not work after a histedi'
   [1]
 
   $ cd ..
+  $ hg clone -qr0 r r0
+  $ cd r0
+  $ echo edit cb9a9f314b8b a > $EDITED
+  $ hg phase -fdr0
+  $ HGEDITOR="cat \"$EDITED\" > " hg histedit 0 2>&1
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  adding a
+  abort: Make changes as needed, you may commit or record as needed now.
+  When you are finished, run hg histedit --continue to resume.
+  [255]
+  $ HGEDITOR=true hg histedit --continue
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  saved backup bundle to $TESTTMP/r0/.hg/strip-backup/cb9a9f314b8b-backup.hg (glob)