Update "arc diff" to amend non-head commits with Mercurial

Summary:
After arc diff creates a revision in Phabricator it amends the commit to include a link to the revision in the commit message. For Mercurial this is done with hg commit --amend --logfile however this will fail when trying to create a diff for a non-head commit.

This updates ArcanistMercurialAPI::amendCommit() to allow amending a non-head commit in two ways, depending on whether evolve is in use:

No evolve:

1. Rebasing the current commit onto the current commit's parent, using the new commit message
2. Rebasing all children + descendants of the current commit onto the new resulting commit
3. Stripping the original commit

With evolve:

1. Amend the commit with hg amend --logfile
2. Run hg evolve to tidy up all commits

Test Plan:
I created 6 commits in a row placing a bookmark at commits 2 bookmark1, 4 bookmark2, and 6 bookmark3, and ensured I had arc:bookmark in my base ruleset.

No evolve, non-head changeset:

1. I verified I did not have evolve enabled by running hg debugextensions and did not see evolve in the listed active extensions.
2. I updated to bookmark1 and modified a file to leave a dirty working state.
3. I ran arc diff and when prompted to amend my changes I said "yes", and verified a phab revision was created properly.
4. I checked the status of my repository and verified it was still linear and the bookmarks pointed to the proper commits.
5. I ran hg log -r bookmark1 --template {desc} to view the full commit message and verified it contained both Summary: ... and Differential Revision: https://....
6. I ran hg diff -c bookmark1 and verified the changes for that commit included the changes I made in step 2.

No evolve, head changeset:

1. I updated to bookmark3 which is the head commit and modified a file to leave a dirty working state.
2. I ran arc diff and when prompted to amend my changes I said "yes", and verified a phab revision was created properly.
3. I checked the status of my repository and verified it was still linear and all the bookmarks pointed to the proper commits.
4. I ran hg log -r bookmark3 --template {desc} to view the full commit message and verified it contained both Summary: ... and Differential Revision: https://....
5. I ran hg diff -c bookmar3 and verified the changes for that commit included the changes I made in step 1.

With evolve:

1. I enabled evolve and verified it was enabled by running hg debugextensions and saw evolve in the listed active extensions.
2. I updated to bookmark2 and modified a file to leave a dirty working state.
3. I ran arc diff and when prompted to amend my changes I said "yes", and verified a phab revision was created properly.
4. I checked the status of my repository and verified it was still linear and all the bookmarks pointed to the proper commits.
5. I ran hg log -r bookmark2 --template {desc} to view the full commit message and verfieid it contained both Summary: ... and Differential Revision: https://....
6. I ran hg diff -c bookmark2 and verified the changes for that commit included the changes I made in step 2.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D21686