progress.c: avoid use of dynamic-sized array

progress.c: avoid use of dynamic-sized array

Dynamically sized arrays are gcc and C99 construct.  Using them hurts
portability to older compilers, although using them is nice in this case
it is not desirable.  This patch removes the only use of the construct
in stop_progress_msg(); the function is about writing out a single line
of a message, and the existing callers of this function feed messages
of only bounded size anyway, so use of dynamic array is simply overkill.

Signed-off-by: Boyd Lynn Gerber <gerberb@zenez.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>