pick/core: Add a method for updating commits

This is almost the same as the one in the ui, but without the UI
elements. It would be nice to share code between them, but I'm not sure
how to do that yet.

bin/pick/core.py

@@ -22,6 +22,7 @@
 
 import asyncio
 import enum
+import itertools
 import json
 import pathlib
 import re
@@ -372,6 +373,25 @@ async def gather_commits(version: str, previous: typing.List['Commit'],
     return commits
 
 
+async def update_commits() -> None:
+    """Gather all new commits and update the on-disk cache.
+    """
+    commits = load()
+    with open('VERSION', 'r') as f:
+        version = '.'.join(f.read().split('.')[:2])
+    if commits:
+        sha = commits[0].sha
+    else:
+        sha = f'{version}-branchpoint'
+
+    if new := await get_new_commits(sha):
+        collected_commits = await gather_commits(version, commits, new)
+    else:
+        collected_commits = []
+
+    save(itertools.chain(collected_commits, commits))
+
+
 def load() -> typing.List['Commit']:
     if not pick_status_json.exists():
         return []