gettext.c: remove/reword a mostly-useless comment

gettext.c: remove/reword a mostly-useless comment

Mostly remove the comment I added 5e9637c6297 (i18n: add
infrastructure for translating Git with gettext, 2011-11-18). Since
then we had a fix in 9c0495d23e6 (gettext.c: detect the vsnprintf bug
at runtime, 2013-12-01) so we're not running with the "set back to C
locale" hack on any modern system.

So having more than 1/4 of the file taken up by a digression about a
glibc bug that mostly doesn't happen to anyone anymore is just a
needless distraction. Shorten the comment to make a brief mention of
the bug, and where to find more info by looking at the git history for
this now-removed comment.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>