convert: p4: ignore purged files with p4d 2012.2 and later

Perforce has the concept of "+Sn" files where only the last revisions of the
file is stored. In p4d 2012.1 old purged revisions were not included in the
"manifest". With 2012.2 they started being included and convert's getfile
failed to recognize the "purged" flag and saw it as an empty file. That made
test-convert-p4-filetypes.t fail.

There is no point in storing an empty file as placeholder for a purged file so
we restore the old behaviour by checking the flag and letting getfile consider
purged files deleted.

(It is questionable whether it makes sense to convert not-yet-purged +S files
to mercurial ... but that is another question.)

hgext/convert/p4.py

@@ -164,6 +164,8 b' class p4_source(converter_source):'
                 raise IOError(d["generic"], data)
 
             elif code == "stat":
+                if d.get("action") == "purge":
+                    return None, None
                 p4type = self.re_type.match(d["type"])
                 if p4type:
                     mode = ""