tags: read tag info into a sorted dict (rather than into a regular dict)

This lets us preserve the original tag order (as specified on the .hgtags file
that is being read). This will be useful to preserve the tag order when saving
the result of a successful automated .hgtags merge (which will be introduced on
a future patch).

There shouldn't be much impact on performance because the sortdict that
_readtags returns is then used to update the alltags regular dict (which
remains a regular dict).

mercurial/tags.py

@@ -12,6 +12,7 b''
 
 from node import nullid, bin, hex, short
 from i18n import _
+import util
 import encoding
 import error
 import errno
@@ -81,7 +82,7 b' def _readtags(ui, repo, lines, fn, recod'
     ids previously associated with it (in file order).  All node ids are
     binary, not hex.'''
 
-    filetags = {}               # map tag name to (node, hist)
+    filetags = util.sortdict()  # map tag name to (node, hist)
     count = 0
 
     def warn(msg):