cache: flush stdio before restoring FDs

cache: flush stdio before restoring FDs

As described in commit 2efb59e (ui-patch: Flush stdout after outputting
data, 2014-06-11), we need to ensure that stdout is flushed before
restoring the file descriptor when writing to the cache.  It turns out
that it's not just ui-patch that is affected by this but also raw diff
which writes to stdout internally.

Let's avoid risking more places doing this by ensuring that stdout is
flushed after writing in fill_slot().

Signed-off-by: John Keeping <john@keeping.me.uk>