Removing the multiple calls to determine VCS type

This removed the subshell from _determine_vcs_base_and_type so it can
set environment variables.

Because this always runs at the beginning of the scripts, there's no
need to do checking if REPOBASE is unset or if VCS_TYPE is not yet
determined, thus I simplified one function and eliminated which_vcs.

bin/blackbox_register_new_file

@@ -47,4 +47,4 @@ vcs_add "$BB_FILES" "$encrypted_file"
 vcs_commit "registered in blackbox: ${unencrypted_file}" "$BB_FILES" "$encrypted_file"
 echo "========== UPDATING VCS: DONE"
 echo "Local repo updated.  Please push when ready."
-echo "    $(which_vcs) push"
+echo "    $VCS_TYPE push"