Use same-directory include for public headers

Use same-directory include for public headers

It doesn't make sense to use "git/somefile.h" in the
public git headers, as it's quite likely that projects
using them will have a git directory themselves. This
alters it, making the public headers look for headers
in the same directory they themselves are in.

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