help: remove repeated word in 'hg help rebase'

Specifically, the second 'with' in 'with which to merge with'.

Differential Revision: https://phab.mercurial-scm.org/D6483

mercurial/commands.py

@@ -3991,7 +3991,7 b' def merge(ui, repo, node=None, **opts):'
     If no revision is specified, the working directory's parent is a
     head revision, and the current branch contains exactly one other
     head, the other head is merged with by default. Otherwise, an
-    explicit revision with which to merge with must be provided.
+    explicit revision with which to merge must be provided.
 
     See :hg:`help resolve` for information on handling file conflicts.