encoding: initialize jsonmap when module is loaded

This makes jsonescape() a thread-safe function, which is necessary for hgweb.
The initialization stuff isn't that slow:

  $ python -m timeit -n1000 -s 'from mercurial import encoding as x' 'reload(x)'
  original:   1000 loops, best of 3: 158 usec per loop
  this patch: 1000 loops, best of 3: 214 usec per loop

compared to loading the commands module:

  $ python -m timeit -n1000 -s 'from mercurial import commands as x' 'reload(x)'
  1000 loops, best of 3: 1.11 msec per loop

mercurial/encoding.py

@@ -379,6 +379,16 b' class normcasespecs(object):'
     other = 0
 
 _jsonmap = []
+_jsonmap.extend("\\u%04x" % x for x in xrange(32))
+_jsonmap.extend(chr(x) for x in xrange(32, 256))
+_jsonmap[0x7f] = '\\u007f'
+_jsonmap[0x09] = '\\t'
+_jsonmap[0x0a] = '\\n'
+_jsonmap[0x22] = '\\"'
+_jsonmap[0x5c] = '\\\\'
+_jsonmap[0x08] = '\\b'
+_jsonmap[0x0c] = '\\f'
+_jsonmap[0x0d] = '\\r'
 
 def jsonescape(s):
     '''returns a string suitable for JSON
@@ -407,18 +417,6 b' def jsonescape(s):'
     ''
     '''
 
-    if not _jsonmap:
-        _jsonmap.extend("\\u%04x" % x for x in xrange(32))
-        _jsonmap.extend(chr(x) for x in xrange(32, 256))
-        _jsonmap[0x7f] = '\\u007f'
-        _jsonmap[0x09] = '\\t'
-        _jsonmap[0x0a] = '\\n'
-        _jsonmap[0x22] = '\\"'
-        _jsonmap[0x5c] = '\\\\'
-        _jsonmap[0x08] = '\\b'
-        _jsonmap[0x0c] = '\\f'
-        _jsonmap[0x0d] = '\\r'
-
     return ''.join(_jsonmap[x] for x in bytearray(toutf8b(s)))
 
 _utf8len = [0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 3, 4]