encoding: add hfsignoreclean to clean out HFS-ignored characters

According to Apple Technote 1150 (unavailable from Apple as far as I
can tell, but archived in several places online), HFS+ ignores sixteen
specific unicode runes when doing path normalization. We need to
handle those cases, so this function lets us efficiently strip the
offending characters from a UTF-8 encoded string (which is the only
way it seems to matter on OS X.)

mercurial/encoding.py

@@ -8,6 +8,28 b''
 import error
 import unicodedata, locale, os
 
+# These unicode characters are ignored by HFS+ (Apple Technote 1150,
+# "Unicode Subtleties"), so we need to ignore them in some places for
+# sanity.
+_ignore = [unichr(int(x, 16)).encode("utf-8") for x in
+           "200c 200d 200e 200f 202a 202b 202c 202d 202e "
+           "206a 206b 206c 206d 206e 206f feff".split()]
+# verify the next function will work
+assert set([i[0] for i in _ignore]) == set(["\xe2", "\xef"])
+
+def hfsignoreclean(s):
+    """Remove codepoints ignored by HFS+ from s.
+
+    >>> hfsignoreclean(u'.h\u200cg'.encode('utf-8'))
+    '.hg'
+    >>> hfsignoreclean(u'.h\ufeffg'.encode('utf-8'))
+    '.hg'
+    """
+    if "\xe2" in s or "\xef" in s:
+        for c in _ignore:
+            s = s.replace(c, '')
+    return s
+
 def _getpreferredencoding():
     '''
     On darwin, getpreferredencoding ignores the locale environment and