archive: simplify archive format guessing

archive: simplify archive format guessing

The code to guess an output archive's format consumed any --format
options and built a new one.  Jonathan noticed that it does so in an
unsafe way, risking to overflow the static buffer fmt_opt.

Change the code to keep the existing --format options intact and to only
add a new one if a format could be guessed based on the output file name.
The new option is added as the first one, allowing the existing ones to
overrule it, i.e. explicit --format options given on the command line win
over format guesses, as before.

To simplify the code further, format_from_name() is changed to return the
full --format option, thus no potentially dangerous sprintf() calls are
needed any more.

Reported-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>