Infer branch name from project name

Since `updatebot` now processes multiple projects in a single run, it no
longer makes sense to have the branch name specified as a command-line
argument.  A unique name is needed for each project, in order to create
separate pull requests for each.  Thus, we have to infer the branch name
from the project name.

updatebot.py

@@ -268,7 +268,6 @@ class Config(pydantic.BaseModel):
 
 class Arguments:
     config: Path
-    branch_name: str
     dry_run: bool
     projects: list[str]
 
@@ -304,7 +303,6 @@ def parse_args() -> Arguments:
         type=Path,
         default=XDG_CONFIG_HOME / 'updatebot' / 'config.toml',
     )
-    parser.add_argument('--branch-name', '-b', default='updatebot')
     parser.add_argument('--dry-run', '-n', action='store_true', default=False)
     parser.add_argument('projects', metavar='project', nargs='*', default=[])
     return parser.parse_args(namespace=Arguments())
@@ -351,8 +349,9 @@ def main() -> None:
         repo_url = config.repo.get_git_url()
         repo = git.Repo.clone_from(repo_url, d, depth=1, b=config.repo.branch)
         for project in config.projects:
-            log.debug('Checking out new branch: %s', args.branch_name)
-            repo.heads[0].checkout(force=True, B=args.branch_name)
+            branch_name = f'updatebot/{project.name}'
+            log.debug('Checking out new branch: %s', branch_name)
+            repo.heads[0].checkout(force=True, B=branch_name)
             title = None
             description = None
             if project.name not in projects:
@@ -370,20 +369,20 @@ def main() -> None:
             if not args.dry_run:
                 repo.head.reference.set_tracking_branch(
                     git.RemoteReference(
-                        repo, f'refs/remotes/origin/{args.branch_name}'
+                        repo, f'refs/remotes/origin/{branch_name}'
                     )
                 )
                 repo.remote().push(force=True)
                 config.repo.create_pr(
                     title,
-                    args.branch_name,
+                    branch_name,
                     config.repo.branch,
                     description,
                 )
             else:
                 print(
                     'Would create PR',
-                    f'{args.branch_name} → {config.repo.branch}:',
+                    f'{branch_name} → {config.repo.branch}:',
                     title,
                 )
                 print(description or '')