Use committer date for commit date and year

Quoting CMakeLists.txt: "[...] of latest commit"

"Latest commit" is better served by using the committer date %cd
instead of the author date %ad: The author date could be some time
last year with committer date being today.

Note this affects both autotools and cmake buildsystems to keep
the generated version message dates and year in sync.

CMakeLists.txt

@@ -69,14 +69,14 @@ if(Git_FOUND)
         )
 
     execute_process(
-        COMMAND "${GIT_EXECUTABLE}" log -1 --format=%ad --date=format:%Y%m%d
+        COMMAND "${GIT_EXECUTABLE}" log -1 --format=%cd --date=format:%Y%m%d
         WORKING_DIRECTORY "${PROJECT_SOURCE_DIR}"
         OUTPUT_VARIABLE GIT_COMMIT_DATE
         ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE
         )
 
     execute_process(
-        COMMAND "${GIT_EXECUTABLE}" log -1 --format=%ad --date=format:%Y
+        COMMAND "${GIT_EXECUTABLE}" log -1 --format=%cd --date=format:%Y
         WORKING_DIRECTORY "${PROJECT_SOURCE_DIR}"
         OUTPUT_VARIABLE GIT_COMMIT_YEAR
         ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE

src/build-helpers/package-version

@@ -71,7 +71,7 @@ if test -d "$GIT_DIR"; then
         echo "version_error_71" | ${TR-tr} -d '\012'; \
         exit 2; \
     }
-    GIT_COMMIT_DATE="$(${GIT-git} log -1 --format=%ad --date=format:%Y%m%d)" || { \
+    GIT_COMMIT_DATE="$(${GIT-git} log -1 --format=%cd --date=format:%Y%m%d)" || { \
         echo "$prog: Error: Cannot run 'git log' for commit date" >&2; \
         echo "version_error_72" | ${TR-tr} -d '\012'; \
         exit 2; \