Added autopatch script for patching and rebasing phabricator diffs

Summary:
This is the first step in a pipeline like the following: autopatch (+rebase) -> build/test -> land. This was inspired after feedback on the land-bot script in D4195.

Reviewer note: I would prefer to split this script into two parts that patch and rebase separately, but this process is intertwined due to the way arcanist handles patches that build on a stack of changes.

Example:
You have two changes A and B, where B builds on A which builds on top of master (currently commit M):

M (master) <- A <- B

After succesful review, you land A. Phabricator has the state stored as such:

M <- A' (master)
  \-- A <- B

The landed A is shown as A' because the hash has changed, but the contents have not.

With a clean source tree on latest master, you arc patch Dxxx and arcanist will fail to find
hash A despite A' being present. You add --skip-dependencies because you know A' is effectively A
and the patch succeeds, but it builds on latest master without prompting. This means that if someone
landed changes before you like:

M <- A' <- C <- B'

The B' will be the result of arc patch, as it will not attempt to search for the dependency A
which doesn't exist on your machine (and because you specified --skip-dependencies to get around
not finding it in the first place).

For this reason, it doesn't make sense to split the script apart, as the preparation steps for a clean arc patch
effectively trigger a rebase under circumstances similar to the above. The final git rebase at the end handles
the remaining cases where the dependency of the patch being applied is available (usually part of master directly).

Test Plan:

test-autopatch.sh
ninja check-arcanist
ninja check
ninja check-all

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4279