Merge branch 'jk/subtree-prefix'

A stray environment variable $prefix could have leaked into and
affected the behaviour of the "subtree" script.

* jk/subtree-prefix:
  subtree: initialize "prefix" variable

contrib/subtree/git-subtree.sh

@@ -46,6 +46,7 @@ ignore_joins=
 annotate=
 squash=
 message=
+prefix=
 
 debug()
 {