bootstrap: don't use `--merges` to find commits
bootstrap: don't use `--merges` to find commits

Shallow clones can cause `git rev-list --merges` to miss merge
commits. Omit it, because the most recent bors commit is
almost always a merge commit.

aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust