compare-disco: support for `file` nodes specification

This leverage the `nodefromfile` feature in core. This make it possible for
callers to no longer pay the subset computation cost (and to make sure the
subset is the right one, even when the base repository is different)

contrib/perf-utils/compare-discovery-case

@@ -99,7 +99,11 b' assert set(VARIANTS.keys()) == set(VARIA'
 
 def parse_case(case):
     case_type, case_args = case.split('-', 1)
-    case = (case_type,) + tuple(int(x) for x in case_args.split('-'))
+    if case_type == 'file':
+        case_args = (case_args,)
+    else:
+        case_args = tuple(int(x) for x in case_args.split('-'))
+    case = (case_type,) + case_args
     return case
 
 
@@ -115,6 +119,8 b' def to_revsets(case):'
         return '::randomantichain(all(), "%d")' % case[1]
     elif t == 'rev':
         return '::%d' % case[1]
+    elif t == 'file':
+        return '::nodefromfile("%s")' % case[1]
     else:
         assert False