Fix case when git root is passed (as dot or .. from deeper)

In that case (i.e. , the `${full#git_root}` would not work and print the full
basically full absolute path to git root.

dotfiles/gittum/bin/slurp

     local path="$1"
     test -n "$GIT_PREFIX" && path="$GIT_PREFIX/$path"
     local full=$(readlink -m "$path")
+    test "$full" = "$git_root" && echo . && return
     printf %s "${full#$git_root/}"
 }