graft: default `base` argument to common case of `ctx.p1()`

I also updated the callers that wanted that, partly to simplify and
partly to show that it works.

Differential Revision: https://phab.mercurial-scm.org/D8027

hgext/histedit.py

@@ -649,7 +649,7 b' def applychanges(ui, repo, ctx, opts):'
             repo.ui.setconfig(
                 b'ui', b'forcemerge', opts.get(b'tool', b''), b'histedit'
             )
-            stats = mergemod.graft(repo, ctx, ctx.p1(), [b'local', b'histedit'])
+            stats = mergemod.graft(repo, ctx, labels=[b'local', b'histedit'])
         finally:
             repo.ui.setconfig(b'ui', b'forcemerge', b'', b'histedit')
     return stats

mercurial/merge.py

@@ -2592,7 +2592,7 b' def update('
 def graft(
     repo,
     ctx,
-    base,
+    base=None,
     labels=None,
     keepparent=False,
     keepconflictparent=False,
@@ -2607,7 +2607,7 b' def graft('
     renames/copies appropriately.
 
     ctx - changeset to rebase
-    base - merge base, usually ctx.p1()
+    base - merge base, or ctx.p1() if not specified
     labels - merge labels eg ['local', 'graft']
     keepparent - keep second parent if any
     keepconflictparent - if unresolved, keep parent used for the merge
@@ -2621,6 +2621,7 b' def graft('
     # which local deleted".
     wctx = wctx or repo[None]
     pctx = wctx.p1()
+    base = base or ctx.p1()
     mergeancestor = repo.changelog.isancestor(pctx.node(), ctx.node())
 
     stats = update(

mercurial/shelve.py

@@ -996,7 +996,6 b' def _rebaserestoredcommit('
         stats = merge.graft(
             repo,
             shelvectx,
-            shelvectx.p1(),
             labels=[b'shelve', b'working-copy'],
             keepconflictparent=True,
         )