git-subtree.sh - regression introduced by da949cc55

Hello,

Up to this commit, I was able to split out new trees out of sub-directories
of my project.

I used this to create a subproject out of a directory, without necessarily
knowing in advance that I could need the content of this directory in other
projects.

Since da949cc55, older unrelated commits get prepended to the history.

This issue is quite easy to reproduce:

 1. choose a directory in your project which hasn't been added with
    "git subtree add". Let's say it's called my/directory/.
 2. "git subtree split --prefix=my/directory --branch the-new-tree"
 3. "git log --stat the-new-tree"
 4. notice the mess which happens at the first commit concerning
    my/directory/ and how it then includes random older commits which have
    nothing related to my/directory.

Jakub, I'm not sure I understand what this patch is supposed to fix. Could
you provide an example ? Or a test case ?

Thanks !
Marc