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_update_all_files

@@ -60,7 +60,6 @@ while IFS= read <&99 -r unencrypted_file; do
 done 99<"$BB_FILES"
 vcs_commit 'Re-encrypted keys'
 
-VCSCMD=$(which_vcs)
 echo '========== DONE.'
 echo 'Likely next step:'
-echo "    ${VCSCMD} push"
+echo "    $VCS_TYPE push"