`git meta rebase` is pretty slow for big jumps

I tried to resurrect an old branch and it took 27 seconsd to rebase:
```
abliss@home:~/src/meta2$ time git meta rebase upstream/master
Rewinding to 7e47a64645a437938ad3c3b31ebde5fc10beb60f.
Applying 096b753f15899a2d5814413f944af95b0f045294.
real 0m27s
$ git log --ancestry-path --oneline 096b753f15899a2d5814413f944af95b0f045294^...HEAD | wc -l
9492
```
The change itself is very simple, so simply checking out `upstream/master` and cherry-picking would have been pretty much instantaneous. (I have 13/6k submodules opened.)