Move public headers to src/git

Move public headers to src/git

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>