churn: compute padding with unicode strings

Most UTF-8 aware terminals convert multibyte sequences into a single displayed
characters.  Because the first column is padded by counting bytes, the second
column is not perfectly aligned in the presence of non ASCII characters.

--HG--
branch : stable
obsolete: fix language and grammar in module docstring

--HG--
branch : stable
extra : amend_source : bd74c1aa540c82f965e68a986337ebe74f58cfce
obsolete: let N denote number of obsoleted changestes in a marker

The number of obsoleted changesets is referred to as N later in the
docstring.

--HG--
branch : stable
obsolete: fix one-element tuple in module docstring

--HG--
branch : stable