When auto-hinting, also try a "minimal" package set

The minimal set is comprised of only the first level of (reverse)
dependencies, before any further iterations of packages are added to
the set.  In some cases, the result of the full iteration will contain
packages which cause problems when migrated but the minimal set,
although possibly a less optimal solution, may be able to migrate
successfully.

It is assumed that migrating the larger set of packages will be
preferred if possible, so minimal sets are tried later.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -2573,6 +2573,7 @@ class Britney:
 
         # loop on them
         candidates = []
+        mincands = []
         for e in excuses:
             excuse = excuses[e]
             if e in self.sources['testing'] and self.sources['testing'][e][VERSION] == excuse.ver[1]:
@@ -2583,29 +2584,34 @@ class Britney:
                     candidates.append(hint.items())
             else:
                 items = [ (e, excuse.ver[1]) ]
+                looped = False
                 for item, ver in items:
                     # excuses which depend on "item" or are depended on by it
                     items.extend( [ (x, excuses[x].ver[1]) for x in excuses if \
                        (item in excuses[x].deps or x in excuses[item].deps) \
                        and (x, excuses[x].ver[1]) not in items ] )
+                    if not looped and len(items) > 1:
+                        mincands.append(items[:])
+                    looped = True
                 if len(items) > 1:
                     candidates.append(items)
 
-        to_skip = []
-        for i in range(len(candidates)):
-            for j in range(i+1, len(candidates)):
-                if i in to_skip or j in to_skip:
-                    # we already know this list isn't interesting
-                    continue
-                elif frozenset(candidates[i]) >= frozenset(candidates[j]):
-                    # j is a subset of i; ignore it
-                    to_skip.append(j)
-                elif frozenset(candidates[i]) <= frozenset(candidates[j]):
-                    # i is a subset of j; ignore it
-                    to_skip.append(i)
-        for i in range(len(candidates)):
-            if i not in to_skip:
-                self.do_hint("easy", "autohinter", [ HintItem("%s/%s" % (x[0], x[1])) for x in candidates[i] ])
+        for l in [ candidates, mincands ]:
+            to_skip = []
+            for i in range(len(l)):
+                for j in range(i+1, len(l)):
+                    if i in to_skip or j in to_skip:
+                        # we already know this list isn't interesting
+                        continue
+                    elif frozenset(l[i]) >= frozenset(l[j]):
+                        # j is a subset of i; ignore it
+                        to_skip.append(j)
+                    elif frozenset(l[i]) <= frozenset(l[j]):
+                        # i is a subset of j; ignore it
+                        to_skip.append(i)
+            for i in range(len(l)):
+                if i not in to_skip:
+                    self.do_hint("easy", "autohinter", [ HintItem("%s/%s" % (x[0], x[1])) for x in l[i] ])
 
     def old_libraries(self):
         """Detect old libraries left in testing for smooth transitions