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.

Conflicts:
	bin/blackbox_deregister_file

I found this easier to just cherry pick since there was a merge and the
merge was reverted.

bin/blackbox_deregister_file

@@ -8,7 +8,6 @@
 
 set -e
 source "${0%/*}/_blackbox_common.sh"
-_determine_vcs_base_and_type
 
 unencrypted_file=$(get_unencrypted_filename "$1")
 encrypted_file=$(get_encrypted_filename "$1")
@@ -31,4 +30,4 @@ vcs_remove "$BB_FILES"
 vcs_commit "Removing from blackbox: ${unencrypted_file}"
 echo "========== UPDATING VCS: DONE"
 echo "Local repo updated.  Please push when ready."
-echo "    $(which_vcs) push"
+echo "    $VCS_TYPE push"