pretty: Fix bug in truncation support for %>, %< and %><

Some systems experience failures in t4205-*.sh (tests 18-20, 27)
which all relate to the use of truncation with the %< padding
placeholder. This capability was added in the commit a7f01c6b
("pretty: support truncating in %>, %< and %><", 19-04-2013).

The truncation support was implemented with the assistance of a
new strbuf function (strbuf_utf8_replace). This function contains
the following code:

       strbuf_attach(sb_src, strbuf_detach(&sb_dst, NULL),
                     sb_dst.len, sb_dst.alloc);

Unfortunately, this code is subject to unspecified behaviour. In
particular, the order of evaluation of the argument expressions
(along with the associated side effects) is not specified by the
C standard. Note that the second argument expression is a call to
strbuf_detach() which, as a side effect, sets the 'len' and 'alloc'
fields of the sb_dst argument to zero. Depending on the order of
evaluation of the argument expressions to the strbuf_attach call,
this can lead to assigning an empty string to 'sb_src'.

In order to remove the undesired behaviour, we replace the above
line of code with:

       strbuf_swap(sb_src, &sb_dst);
       strbuf_release(&sb_dst);

which achieves the desired effect without provoking unspecified
behaviour.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>