teuthology/repo_utils: Optimize repository cloning via bare repositories and worktrees

teuthology/repo_utils: Optimize repository cloning via bare repositories and worktrees

Repeatedly performing full clones of the QA suite and other repositories
for different commits or branches slows down testing iterations and
consumes excess disk space and I/O.

This commit introduces an optimization that initializes a single bare
clone of a target repository. It subsequently utilizes git worktree to
provision lightweight, isolated workspaces for requested commits. This
reduces redundant network fetches and significantly accelerates
repository checkout operations.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>