commands: support passing depth to hg.clone()

This will allow extensions to add --depth or other arguments to control
depth fetching.

Differential Revision: https://phab.mercurial-scm.org/D5164

mercurial/commands.py

@@ -1553,7 +1553,8 b' def clone(ui, source, dest=None, **opts)'
                  branch=opts.get('branch'),
                  shareopts=opts.get('shareopts'),
                  storeincludepats=includepats,
-                 storeexcludepats=excludepats)
+                 storeexcludepats=excludepats,
+                 depth=opts.get('depth') or None)
 
     return r is None