Dirty cache of HEAD commit after an amend/range reload

Summary:
Introducing `--head` caused us to run `git diff base..head` explicitly.

However, we can now hit this workflow:

  - We resolve `HEAD` as commit `aaaa1`.
  - This is cached.
  - We notice dirty working copy changes and prompt the user to amend them to HEAD.
  - The user accepts the amend.
  - We amend, creating commit `bbbb2`.
  - We dirty the commit range and reload the working copy. This //does not// dirty the cache of HEAD.
  - We run `git diff`, but it uses the old cached HEAD: `git diff base..aaaa1`.
  - This works fine (`aaaa1` still exists, it's just not on any branch) but produces the wrong diff (without amended changes).

To resolve this, implement the "dirty the cache when the range reloads" hook.

Also never try to amend if the user provides `--head`.

Test Plan:
Ran `arc diff --only --trace` in a working copy with a new commit and some uncommitted changes.

  - Prior to this change, saw a `git diff base..aaaa1` command and the wrong diff.
  - After this change, saw a `git diff base..bbbb2` command and the correct diff.

Reviewers: chad, csilvers, talshiri

Reviewed By: talshiri

Subscribers: epriestley, spicyj

Differential Revision: https://secure.phabricator.com/D9506