i18n: calculate terminal columns by width information of each characters
neither number of 'bytes' in any encoding nor 'characters' is
appropriate to calculate terminal columns for specified string.
this patch modifies MBTextWrapper for:
- overriding '_wrap_chunks()' to make it use not built-in 'len()'
but 'encoding.colwidth()' for columns of string
- fixing '_cutdown()' to make it use 'encoding.colwidth()' instead
of local, similar but incorrect implementation
this patch also modifies 'encoding.py':
- dividing 'colwith()' into 2 pieces: one for calculation columns of
specified UNICODE string, and another for rest part of original
one. the former is used from MBTextWrapper in 'util.py'.
- preventing 'colwidth()' from evaluating HGENCODINGAMBIGUOUS
configuration per each invocation: 'unicodedata.east_asian_width'
checking is kept intact for reducing startup cost.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null