abspath.c

Currently parse-options.o pulls quite a big bunch of dependencies.
his complicates it's usage in contrib/ because it pulls external
dependencies and it also increases executables size.

Split off less generic and more internal to git part of
parse-options.c to parse-options-cb.c.

Move prefix_filename function from setup.c to abspath.c. abspath.o
and wrapper.o pull each other, so it's unlikely to increase the
dependencies. It was a dependency of parse-options.o that pulled
many others.

Now parse-options.o pulls just abspath.o, ctype.o, strbuf.o, usage.o,
wrapper.o, libc directly and strlcpy.o indirectly.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>