Make irregexp more efficient

SCMProvider.check_for_update will clone the entire v8 repo, despite the fact that we pre-cloned it, so we can run merge-base and related on it.

[task 2024-12-06T18:18:35.641Z] [Debug]  (SCMProvider irregexp job_id=661) ================================================
[task 2024-12-06T18:18:35.641Z] [Info]   (SCMProvider irregexp job_id=661) Beginning SCMProvider.check_for_update
[task 2024-12-06T18:18:35.641Z] [Debug]  (SCMProvider irregexp job_id=661)  Arguments: (<components.scmprovider.SCMProvider object at 0x7ff558951c50>, {{ irregexp  Core  JavaScript Engine  c3e48a7c58d9a88cb46848b59fb1f621c72a9606  https://chromium.googlesource.com/v8/v8.git  False  regular  iireland@mozilla.com  iain  sm-shell  None  None  js/src/irregexp/moz.yaml [{vendoring True None [] [] 1 week linux None None None }] }}, {vendoring True None [] [] 1 week linux None None None }, 'd9f0f66ecb8037e0ef9c38861fdbe4e95113e56a', <Job id: 654 library: irregexp>) {}
[task 2024-12-06T18:18:35.642Z] [Debug]  (CommandProvider irregexp job_id=661) ----------------------------------------------
[task 2024-12-06T18:18:35.642Z] [Info]   (CommandProvider irregexp job_id=661) Running ['git', 'clone', 'https://chromium.googlesource.com/v8/v8.git', '.']
[task 2024-12-06T18:20:00.847Z] [Info]   (CommandProvider irregexp job_id=661) Return: 0 Runtime (s): 85
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) -------
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) stdout:
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) 
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) -------
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) stderr:
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) Cloning into '.'...
[task 2024-12-06T18:20:00.847Z] Updating files:  88% (14993/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  89% (15022/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  90% (15191/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  91% (15359/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  92% (15528/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  93% (15697/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  94% (15866/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  95% (16035/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  96% (16203/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  97% (16372/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  98% (16541/16878)
[task 2024-12-06T18:20:00.847Z] Updating files:  99% (16710/16878)
[task 2024-12-06T18:20:00.847Z] Updating files: 100% (16878/16878)
[task 2024-12-06T18:20:00.847Z] Updating files: 100% (16878/16878), done.
[task 2024-12-06T18:20:00.847Z] 
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) ----------------------------------------------
[task 2024-12-06T18:20:00.847Z] [Debug]  (CommandProvider irregexp job_id=661) ----------------------------------------------
[task 2024-12-06T18:20:00.847Z] [Info]   (CommandProvider irregexp job_id=661) Running ['git', 'merge-base', 'c3e48a7c58d9a88cb46848b59fb1f621c72a9606', 'd9f0f66ecb8037e0ef9c38861fdbe4e95113e56a']
[task 2024-12-06T18:20:00.851Z] [Info]   (CommandProvider irregexp job_id=661) Return: 0 Runtime (s): 0
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) -------
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) stdout:
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) c3e48a7c58d9a88cb46848b59fb1f621c72a9606
[task 2024-12-06T18:20:00.851Z] 
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) -------
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) stderr:
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) 
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) ----------------------------------------------
[task 2024-12-06T18:20:00.851Z] [Debug]  (SCMProvider irregexp job_id=661) Our common ancestor is c3e48a7c58d9a88cb46848b59fb1f621c72a9606.
[task 2024-12-06T18:20:00.851Z] [Debug]  (CommandProvider irregexp job_id=661) ----------------------------------------------

We should do something where Updatebot gets told there is a pre-seeded checkout and uses that instead of doing a whole clone

Start at SCMProvider.check_for_update and trace how the pre-seeded checkout is passed to Updatebot and how the current git clone and merge-base commands are invoked. Confirm the existing checkout can replace the full clone, then verify that merge-base still runs against the intended revisions and that the clone is no longer performed.