split: use ctx.rev() instead of %d % ctx

And rename a variable for clarity while we're here.

This is part of our effort to wean off basectx.__int__.

Differential Revision: https://phab.mercurial-scm.org/D2430

hgext/split.py

@@ -173,6 +173,6 b' def dosplit(ui, repo, tr, ctx, opts):'
 
     return committed[-1]
 
-def dorebase(ui, repo, src, dest):
+def dorebase(ui, repo, src, destctx):
     rebase.rebase(ui, repo, rev=[revsetlang.formatspec('%ld', src)],
-                  dest=revsetlang.formatspec('%d', dest))
+                  dest=revsetlang.formatspec('%d', destctx.rev()))