import-checker: make search algorithm non-recursive breadth-first

Breadth-first allows finding the shortest cycle including the starting
module. This lets us terminate our search early when we've discovered
shorter paths already. This gives a tremendous speed-up to the
cycle-finding portion of the test, dropping total runtime from 39s to
3s.

contrib/import-checker.py

@@ -166,22 +166,21 b' class CircularImport(Exception):'
 def cyclekey(names):
     return tuple(sorted(names))
 
-def check_one_mod(mod, imports, path=None, ignore=None):
-    if path is None:
-        path = []
-    if ignore is None:
-        ignore = []
-    path = path + [mod]
-    for i in sorted(imports.get(mod, [])):
-        if i not in stdlib_modules and not i.startswith('mercurial.'):
-            i = mod.rsplit('.', 1)[0] + '.' + i
-        if i in path:
-            firstspot = path.index(i)
-            cycle = path[firstspot:]
-            if cyclekey(cycle) not in ignore:
-                raise CircularImport(cycle)
-            continue
-        check_one_mod(i, imports, path=path, ignore=ignore)
+def checkmod(mod, imports):
+    shortest = {}
+    visit = [[mod]]
+    while visit:
+        path = visit.pop(0)
+        for i in sorted(imports.get(path[-1], [])):
+            if i not in stdlib_modules and not i.startswith('mercurial.'):
+                i = mod.rsplit('.', 1)[0] + '.' + i
+            if len(path) < shortest.get(i, 1000):
+                shortest[i] = len(path)
+                if i in path:
+                    if i == path[0]:
+                        raise CircularImport(path)
+                    continue
+                visit.append(path + [i])
 
 def rotatecycle(cycle):
     """arrange a cycle so that the lexicographically first module listed first
@@ -207,7 +206,7 b' def find_cycles(imports):'
     cycles = {}
     for mod in sorted(imports.iterkeys()):
         try:
-            check_one_mod(mod, imports, ignore=cycles)
+            checkmod(mod, imports)
         except CircularImport, e:
             cycle = e.args[0]
             cycles[cyclekey(cycle)] = ' -> '.join(rotatecycle(cycle))