Rollup merge of #131358 - onur-ozkan:129528, r=Mark-Simulacrum

force "HEAD" for non-CI and `git_upstream_merge_base` for CI environment

When rust-lang/rust is configured as remote, some of the git logic (for tracking changed files) that uses get_closest_merge_commit starts to produce annoying results as the upstream branch becomes outdated quickly (since it isn't updated with git pull). We can rely on HEAD for non-CI environments as we specifically treat bors commits as merge commits, which also exist on upstream. As for CI environments, we should use `git_upstream_merge_base` to correctly track modified files as bors commits may be in `HEAD` but not yet on the upstream remote.

This is also an alternative fix for rust-lang/rust#129528 since rust-lang/rust#131331 reverts the previous fix attempts.