add "vcs" to pass commands to the vcs program

This new command allows the user to execute arbitrary commands inside the
environment etckeeper runs in. This ensures that variables like $HOME match
the values present during the commit process.

Using the command "etckeeper vcs config user.email" will use the same git
configuration as "etckeeper commit", while the value used by "git config"
might vary due to the use of "sudo" and its handling of $HOME.

vcs.d/30vcs-cmd

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+IFS='
+'
+
+# check whether we can locate the vcs binary
+if [ -n "$VCS" ] && which "$VCS" > /dev/null; then
+        # pass commands to the VCS application
+        $VCS $@
+fi