vcxproj: let vcs-svn depend on libgit

It really does depend on libgit. It does not hurt to let it depend on
xdiff, and it makes the code simpler.

It is necessary to get this dependency chain right, because we will
introduce a change where the vcpkg system is initialized before building
libgit. The vcpkg system will then build the dependencies needed by Git
(and thereby make the include headers available):

As the vcpkg system cannot be run in parallel (it does not lock,
wreaking havoc with files being accessed and written at the same time,
letting the vcpkg processes stumble over each others' toes. We prevent
that by ensuring that only one project is built at first: libgit. And
this project's PreBuildEvent will be used to initialize vcpkg and build
all dependencies. Subsequently, the other projects can be built in
parallel.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

contrib/buildsystems/Generators/Vcxproj.pm

@@ -296,7 +296,7 @@ EOM
     print F << "EOM";
   </ItemGroup>
 EOM
-    if (!$static_library) {
+    if (!$static_library || $target =~ 'vcs-svn') {
       my $uuid_libgit = $$build_structure{"LIBS_libgit_GUID"};
       my $uuid_xdiff_lib = $$build_structure{"LIBS_xdiff/lib_GUID"};