stringutil: update list of re-special characters to include &~

I missed this because I was looking at the change that refactored
re.escape, and these characters were added in
https://github.com/python/cpython/commit/05cb728d68a278d11466f9a6c8258d914135c96c.

Differential Revision: https://phab.mercurial-scm.org/D3850

mercurial/utils/stringutil.py

@@ -25,7 +25,7 b' from .. import ('
 
 # regex special chars pulled from https://bugs.python.org/issue29995
 # which was part of Python 3.7.
-_respecial = pycompat.bytestr(b'()[]{}?*+-|^$\\.# \t\n\r\v\f')
+_respecial = pycompat.bytestr(b'()[]{}?*+-|^$\\.&~# \t\n\r\v\f')
 _regexescapemap = {ord(i): (b'\\' + i).decode('latin1') for i in _respecial}
 
 def reescape(pat):