perf: fallback to subset if ondisk cache is missing in perfbranchmapload If there is no branchmap on disk for that filter, it means that the cache from some subset's filter is relevant for this one. We look for it instead of aborting. That way it is much simpler to run the command in an automated way. We can now add it to `test-contrib-perf.t`.

--- a/tests/test-contrib-perf.t	Fri Nov 23 02:11:11 2018 +0100
+++ b/tests/test-contrib-perf.t	Fri Nov 23 02:08:01 2018 +0100
@@ -143,6 +143,7 @@
   $ hg perfunidiff --alldata 1
   $ hg perfbookmarks
   $ hg perfbranchmap
+  $ hg perfbranchmapload
   $ hg perfcca
   $ hg perfchangegroupchangelog
   $ hg perfchangegroupchangelog --cgversion 01