cmake(mergetools): better support for out-of-tree builds

In 7e0730c (t: better support for out-of-tree builds, 2024-12-06)
the strategy was changed from letting `t7609-mergetool--lib.sh`
hard-code the directory where it expects to find the merge tools to
hard-coding that value in the placeholder `@GIT_TEST_MERGE_TOOLS_DIR@`
that is replaced during the build.

However, likely due to a copy/paste mistake (and reviewers missed this,
too), the CMake-based build was adjusted incorrectly, replacing that
placeholder not with the path to the merge tools, but with a Boolean
indicating whether to use a runtime-generated path prefix or not.

Let's fix that, addressing this CMake-build's symptom:

  Initialized empty Git repository in D:/a/git/git/t/trash directory.t7609-mergetool--lib/.git/
  ++ . true/vimdiff
  ./test-lib.sh: line 1021: true/vimdiff: No such file or directory

Signed-off-by: Johannes Schindelin <[email protected]>

contrib/buildsystems/CMakeLists.txt

@@ -1195,7 +1195,7 @@ string(REPLACE "@GIT_TEST_TEXTDOMAINDIR@" "'${CMAKE_BINARY_DIR}/po/build/locale'
 string(REPLACE "@GIT_TEST_POPATH@" "'${CMAKE_BINARY_DIR}/po'" git_build_options "${git_build_options}")
 string(REPLACE "@GIT_TEST_TEMPLATE_DIR@" "'${CMAKE_BINARY_DIR}/templates/blt'" git_build_options "${git_build_options}")
 string(REPLACE "@GIT_TEST_GITPERLLIB@" "'${CMAKE_BINARY_DIR}/perl/build/lib'" git_build_options "${git_build_options}")
-string(REPLACE "@GIT_TEST_MERGE_TOOLS_DIR@" "'${RUNTIME_PREFIX}'" git_build_options "${git_build_options}")
+string(REPLACE "@GIT_TEST_MERGE_TOOLS_DIR@" "'${CMAKE_BINARY_DIR}/mergetools'" git_build_options "${git_build_options}")
 string(REPLACE "@RUNTIME_PREFIX@" "'${RUNTIME_PREFIX}'" git_build_options "${git_build_options}")
 string(REPLACE "@GITWEBDIR@" "'${GITWEBDIR}'" git_build_options "${git_build_options}")
 string(REPLACE "@USE_GETTEXT_SCHEME@" "" git_build_options "${git_build_options}")