rebase: don't forward "source" argument to rebase (issue4633)

`hg pull` takes an optional "source" argument to define the path/url to
pull from. Under some circumstances, this option could get proxied to
rebase and interpretted as the --source argument to rebase, leading to
unexpected behavior.

In my local environment, "source" always appears in "opts" in
pullrebase. However, when attempting to write a test, I couldn't reproduce
this. Instead, the source is being captured as a positional argument in
"args." I suspect an interaction between **kwargs and an extension is to
blame for the differences in behavior. This is why no test has been
written.

I have tested behavior locally and the patch has the intended
side-effect of making `hg pull --rebase` work again.

hgext/rebase.py

@@ -1040,6 +1040,10 b' def pullrebase(orig, ui, repo, *args, **'
             # dropping it
             if 'rev' in opts:
                 del opts['rev']
+            # positional argument from pull conflicts with rebase's own
+            # --source.
+            if 'source' in opts:
+                del opts['source']
             rebase(ui, repo, **opts)
             branch = repo[None].branch()
             dest = repo[branch].rev()