Makefile: list one vcs-svn/xdiff object or header per line

Makefile: list one vcs-svn/xdiff object or header per line

As the svn import infrastructure evolves, it's getting to be a pain to
tell by eye what files were added or removed from a dependency line
like

	VCSSVN_OBJS = vcs-svn/string_pool.o vcs-svn/line_buffer.o \
		vcs-svn/repo_tree.o vcs-svn/fast_export.o vcs-svn/svndump.o

So use a style with one entry per line instead, like the existing
BUILTIN_OBJS:

	# protect against environment
	VCSSVN_OBJS =
	...
	VCSSVN_OBJS += vcs-svn/string_pool.o
	VCSSVN_OBJS += vcs-svn/line_buffer.o
	...

which is readable on its own and produces nice, clear diffs.

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