Flush progress message buffer in display().

Flush progress message buffer in display().

This will make progress display from pack-objects (invoked via
upload-pack) more responsive on platforms with an implementation
of stdio whose stderr is line buffered.

The standard error stream is defined to be merely "not fully
buffered"; this is different from "unbuffered".  If the
implementation of the stdio library chooses to make it line
buffered, progress reports that end with CR but do not contain
LF will accumulate in the stdio buffer before written out.  A
fflush() after each progress display gives a nice continuous
progress.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>