The idea is taken from Junio's work in read-cache.c, where
it's used for writing out the index without tap-dancing on
the poor harddrive. Since it's almost certainly useful for
cached writing of packfiles too, we turn it into a generic
API, making it perfectly simple to reuse it later.

gitfo_write_cached() has the same contract as gitfo_write(), it
returns GIT_SUCCESS if all bytes are successfully written (or were
at least buffered for later writing), and <0 if an error occurs
during buffer writing.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>