Use tribits_advanced_set_cache_var_and_default() for <Project>_SHOW_G…

…IT_COMMIT_PARENTS (TriBITSPub#597)

We eventually need to use this for all of those vars to make that code more
compact.

tribits/core/package_arch/TribitsGlobalMacros.cmake

@@ -686,13 +686,8 @@ macro(tribits_define_global_options_and_define_extra_repos)
     CACHE BOOL
     "Generate the ${PROJECT_NAME}RepoVersion.txt file.")
 
-  if ("${${PROJECT_NAME}_SHOW_GIT_COMMIT_PARENTS_DEFAULT}" STREQUAL "")
-    set(${PROJECT_NAME}_SHOW_GIT_COMMIT_PARENTS_DEFAULT OFF)
-  endif()
-  advanced_set(${PROJECT_NAME}_SHOW_GIT_COMMIT_PARENTS
-    ${${PROJECT_NAME}_SHOW_GIT_COMMIT_PARENTS_DEFAULT}
-    CACHE BOOL
-    "Show parents' commit info in the repo version output.")
+  tribits_advanced_set_cache_var_and_default(${PROJECT_NAME}_SHOW_GIT_COMMIT_PARENTS
+    BOOL OFF "Show parents' commit info in the repo version output.")
 
   if ("${${PROJECT_NAME}_GENERATE_VERSION_DATE_FILES_DEFAULT}" STREQUAL "")
     set(${PROJECT_NAME}_GENERATE_VERSION_DATE_FILES_DEFAULT OFF)