util: use set for reserved Windows filenames

Previously, we were performing membership testing against a
list. Change it to a set for a minor perf win. While we're at it,
explode the assignment in place so less work is needed at module
import time.

Differential Revision: https://phab.mercurial-scm.org/D600

mercurial/util.py

@@ -1230,9 +1230,11 b' def copyfiles(src, dst, hardlink=None, p'
 
     return hardlink, num
 
-_winreservednames = b'''con prn aux nul
-    com1 com2 com3 com4 com5 com6 com7 com8 com9
-    lpt1 lpt2 lpt3 lpt4 lpt5 lpt6 lpt7 lpt8 lpt9'''.split()
+_winreservednames = {
+    'con', 'prn', 'aux', 'nul',
+    'com1', 'com2', 'com3', 'com4', 'com5', 'com6', 'com7', 'com8', 'com9',
+    'lpt1', 'lpt2', 'lpt3', 'lpt4', 'lpt5', 'lpt6', 'lpt7', 'lpt8', 'lpt9',
+}
 _winreservedchars = ':*?"<>|'
 def checkwinfilename(path):
     r'''Check that the base-relative path is a valid filename on Windows.