encoding: add fast-path for ASCII uppercase.

This copies the performance hack from encoding.lower (c481761033bd).

The case-folding logic that kicks in on case-insensitive filesystems
hits encoding.upper hard: with a repository with 75k files, the
timings went from

  hg perfstatus
  ! wall 3.156000 comb 3.156250 user 1.625000 sys 1.531250 (best of 3)

to

  hg perfstatus
  ! wall 2.390000 comb 2.390625 user 1.078125 sys 1.312500 (best of 5)

This is a 24% decrease. For comparison, Mercurial 2.0 gives:

  hg perfstatus
  ! wall 2.172000 comb 2.171875 user 0.984375 sys 1.187500 (best of 5)

so we're only 10% slower than before we added the extra case-folding
logic.

The same decrease is seen when executing 'hg status' as normal, where
we go from:

  hg status --time
  time: real 4.322 secs (user 2.219+0.000 sys 2.094+0.000)

to

  hg status --time
  time: real 3.307 secs (user 1.750+0.000 sys 1.547+0.000)

mercurial/encoding.py

@@ -190,6 +190,11 b' def lower(s):'
 def upper(s):
     "best-effort encoding-aware case-folding of local string s"
     try:
+        s.decode('ascii') # throw exception for non-ASCII character
+        return s.upper()
+    except UnicodeDecodeError:
+        pass
+    try:
         if isinstance(s, localstr):
             u = s._utf8.decode("utf-8")
         else: