Added global cmake cache var to show git commit parents (#597)

Added global cmake cache variable `<PROJECT_NAME>_SHOW_GIT_COMMIT_PARENTS:BOOL`
that is default to OFF. This will be a project based variable to including
parent commit info in the repo version output.

tribits/core/package_arch/TribitsGlobalMacros.cmake

@@ -685,6 +685,14 @@ 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.")
+
   if ("${${PROJECT_NAME}_GENERATE_VERSION_DATE_FILES_DEFAULT}" STREQUAL "")
     set(${PROJECT_NAME}_GENERATE_VERSION_DATE_FILES_DEFAULT OFF)
   endif()

tribits/doc/build_ref/TribitsBuildReferenceBody.rst

@@ -2934,6 +2934,20 @@ NOTE: If the base ``.git/`` directory is missing, then no
 printed to cmake STDOUT.
 
 
+Show parent(s) commit info in the repo version output
+----------------------------------------------------
+
+.. _<Project>_SHOW_GIT_COMMIT_PARENTS:
+
+When working with local git repos for the project sources, one can include
+the repo's head commit parent(s) info in the repo version output using::
+
+   -D <Project>_SHOW_GIT_COMMIT_PARENTS=ON
+
+For each parent commit, this will include their SHA1, author name, date, email
+and its 80 character summary message in the repo version output string.
+
+
 Generating git version date files
 ---------------------------------