diff.c: flush stdout before printing rename warnings

diff.c: flush stdout before printing rename warnings

The diff output is buffered in a FILE object and could still be
partially buffered when we print these warnings (directly to fd 2).
The output is messed up like this

 worktree.c                                   |   138 +-
 worktree.h        warning: inexact rename detection was skipped due to too many files.
                           |    12 +-
 wrapper.c                                    |    83 +-

It gets worse if the warning is printed after color codes for the graph
part are already printed. You'll get a warning in green or red.

Flush stdout first, so we can get something like this instead:

 xdiff/xutils.c                               |    42 +-
 xdiff/xutils.h                               |     4 +-
 1033 files changed, 150824 insertions(+), 69395 deletions(-)
warning: inexact rename detection was skipped due to too many files.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>