Makefile: fold XDIFF_H and VCSSVN_H into LIB_H

Makefile: fold XDIFF_H and VCSSVN_H into LIB_H

Just like MISC_H (see previous commit), there is no reason to track
xdiff and vcs-svn headers separately from the rest of the headers.
The only purpose of these variables is to keep track of recompilation
dependencies.

As a pleasant side effect, folding these into LIB_H lets us stop
tracking GIT_OBJS and VCSSVN_TEST_OBJS separately from the list of all
OBJECTS.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>