If someone provides a --prefix that ends with slash, strip the slash.

Prefixes that differ only in the trailing slash should be considered
identical.

Also update the test to check that this works.
diff --git a/git-subtree.sh b/git-subtree.sh
index 454ce7e..0949fef 100755
--- a/git-subtree.sh
+++ b/git-subtree.sh
Binary files differ