xdiff: move xdiff to 'deps'

xdiff is a dependency (from git core) and more properly belongs in the
'deps' directory. Move it there, and add a stub for cmake to resolve
xdiff from the system location in the future. (At present, bundled xdiff
remains hardcoded.)

cmake/SelectXdiff.cmake

@@ -0,0 +1,9 @@
+# Optional external dependency: xdiff
+if(USE_XDIFF STREQUAL "system")
+	message(FATAL_ERROR "external/system xdiff is not yet supported")
+else()
+	add_subdirectory("${PROJECT_SOURCE_DIR}/deps/xdiff" "${PROJECT_BINARY_DIR}/deps/xdiff")
+	list(APPEND LIBGIT2_DEPENDENCY_INCLUDES "${PROJECT_SOURCE_DIR}/deps/xdiff")
+	list(APPEND LIBGIT2_DEPENDENCY_OBJECTS "$<TARGET_OBJECTS:xdiff>")
+	add_feature_info(xdiff ON "xdiff support (bundled)")
+endif()