cmake: quote the path accurately when editing `test-lib.sh`

By default, the build directory will be called something like
`contrib/buildsystems/out/build/x64-Debug (default)` (note the space and
the parentheses). We need to make sure that such a path is quoted
properly when editing the assignment of the `GIT_BUILD_DIR` variable.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/buildsystems/CMakeLists.txt

diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
index 02241dc..3bb7433 100644
--- a/contrib/buildsystems/CMakeLists.txt
+++ b/contrib/buildsystems/CMakeLists.txt
@@ -949,7 +949,7 @@
 	file(WRITE ${CMAKE_BINARY_DIR}/CTestCustom.cmake
 		"file(STRINGS ${CMAKE_SOURCE_DIR}/t/test-lib.sh GIT_BUILD_DIR_REPL REGEX \"GIT_BUILD_DIR=(.*)\")\n"
 		"file(STRINGS ${CMAKE_SOURCE_DIR}/t/test-lib.sh content NEWLINE_CONSUME)\n"
-		"string(REPLACE \"\${GIT_BUILD_DIR_REPL}\" \"GIT_BUILD_DIR=\\\"$TEST_DIRECTORY\\\"/../${BUILD_DIR_RELATIVE}\" content \"\${content}\")\n"
+		"string(REPLACE \"\${GIT_BUILD_DIR_REPL}\" \"GIT_BUILD_DIR=\\\"$TEST_DIRECTORY/../${BUILD_DIR_RELATIVE}\\\"\" content \"\${content}\")\n"
 		"file(WRITE ${CMAKE_SOURCE_DIR}/t/test-lib.sh \${content})")
 	#misc copies
 	file(COPY ${CMAKE_SOURCE_DIR}/t/chainlint.sed DESTINATION ${CMAKE_BINARY_DIR}/t/)