Fix target determination file diffing (pytorch#38661)

Summary:
It seems like all this time this was accidentally doing a 3-way merge-base, oops.
Pull Request resolved: pytorch#38661

Test Plan:
```
$ git checkout gh/mohammadmahdijavanmard/1/head
$ git merge-base origin master HEAD --all
8292742
$ git merge-base origin/master HEAD --all
324dc16
```

Differential Revision: D21640939

Pulled By: yns88

fbshipit-source-id: 0f59922e7c0fd046f48fec30e8aa25c244f6dd62

.jenkins/pytorch/common.sh

@@ -187,7 +187,7 @@ function file_diff_from_base() {
   set +e
   git fetch origin master --quiet
   set -e
-  git diff --name-only "$(git merge-base origin master HEAD)" > "$1"
+  git diff --name-only "$(git merge-base origin/master HEAD)" > "$1"
 }
 
 function get_bazel() {