Use older git syntax

Turns out that git v1.7 does not support `--date=uni`, but `%ct` should
be roughly equivalent.

src/include/facts.sh

@@ -55,7 +55,7 @@ git_fact() {
             git log -1 --pretty=format:%h HEAD
             ;;
         latest_cdate)
-            git log -1 --format=%cd --date=unix HEAD
+            git log -1 --format=%ct HEAD
             ;;
         *)
             warn "unknown git fact asked: $fact_name"