pretty.c: format string with truncate respects logOutputEncoding

Pretty format string %<(N,[ml]trunc)>%s truncates subject to a given
length with an appropriate padding. This works for non-ASCII texts when
i18n.logOutputEncoding is UTF-8 only (independently of a printed commit
message encoding) but does not work when i18n.logOutputEncoding is NOT
UTF-8.

In 7e77df3 (pretty: two phase conversion for non utf-8 commits, 2013-04-19)
'format_commit_item' function assumes commit message to be in UTF-8.
And that was so until ecaee80 (pretty: --format output should honor
logOutputEncoding, 2013-06-26) where conversion to logOutputEncoding was
added before calling 'format_commit_message'.

Correct this by converting a commit message to UTF-8 first (as it
assumed in 7e77df3 (pretty: two phase conversion for non utf-8 commits,
2013-04-19)). Only after that set 'output_enc' variable to an actual
logOutputEncoding.