Fix target determination file diffing (#38661)
Summary:
It seems like all this time this was accidentally doing a 3-way merge-base, oops.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38661
Test Plan:
```
$ git checkout gh/mohammadmahdijavanmard/1/head
$ git merge-base origin master HEAD --all
8292742ba020fcff90f14418c18741ebf606103b
$ git merge-base origin/master HEAD --all
324dc1623e2f91892038fb1b151450a7c6529dd9
```
Differential Revision: D21640939
Pulled By: yns88
fbshipit-source-id: 0f59922e7c0fd046f48fec30e8aa25c244f6dd62
