Makefile: add missing header file dependencies

Makefile: add missing header file dependencies

LIB_H is missing exec_cmd.h and color.h.  cache.h includes
SHA1_HEADER, and thus so does almost everything else, so add that
to LIB_H, too.  xdiff-interface.h is not included by any header
files, but so many source files use xdiff that it is simplest to
include it in LIB_H, too.

xdiff-interface.o uses the xdiff library heavily; let it depend
on all xdiff headers to avoid needing to keep track of which
headers it uses.

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