Add verbose log message pointing to sibling git dir

Adds a log message showing the location of the directory where the actual git prepare is running (but only if `--verbose` is passed).

Might be useful for other users encountering git errors and not being able to reproduce/fix because they are not aware of the "sibling directory" being used. Like was the case here #20.

index.js

@@ -29,6 +29,7 @@ module.exports = {
       },
       prepare: function(context) {
         var d = context.gitDeploy;
+        this.log("preparing git in " + d.worktreePath, { verbose: true });
         return git.prepareTree(d.worktreePath, d.myRepo, d.repo, d.branch);
       },
       upload: function(context) {