R1:426d64868126

In Drydock, don't reset current branch to point at unrelated commit

Summary:
Fixes T10037. When we're building commit `aabbccdd`, we currently do this to check it out:

git reset --hard aabbccdd

However, this has an undesirable side effect of moving the current branch pointer to point at `aabbccdd`. The current branch pointer may be some totally different branch which `aabbccdd` is not part of, so this is confusing and misleading.

Instead, use `git reset --hard HEAD` to get the primary effect we want…