cherry-pick: do not dump core when iconv fails

cherry-pick: do not dump core when iconv fails

When cherry-picking, usually the new and old commit encodings are both
UTF-8.  Most old iconv implementations do not support this trivial
conversion, so on old platforms, out->message remains NULL, and later
attempts to read it segfault.

Fix this by noticing the input and output encodings match and skipping
the iconv step, like the other reencode_string() call sites already do.
Also stop segfaulting on other iconv failures: if iconv fails for some
other reason, the best we can do is to pass the old message through.

This fixes a regression introduced in v1.7.1-rc0~15^2~2 (revert:
clarify label on conflict hunks, 2010-03-20).

Reported-by: Andreas Krey <a.krey@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>