Makefile: rearrange dependency rules

Makefile: rearrange dependency rules

Put rules listing dependencies of compiled objects (.o files) on
header files (.h files) in one place, to make them easier to
compare and modify all at once.

Add a GIT_OBJS variable listing objects that depend on LIB_H,
for similar reasons.

No change in build-time behavior intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>