It's arguably smoother to keep them close to the source,
as that's where one's working when modifying them. More
importantly, though, is the ability to use private headers
in the src/ dir that simply include "git/$samename.h" to
get to the public API at the same time.

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