[jvmakine]

To update a directory that was scaffolded earlier, a tool has to render the templates again exactly as they were back then, then compare that with a render of the latest templates. Open can't do this, because it doesn't report which revision it fetched.

This adds an interface for sources that can reproduce earlier revisions:

type Source interface {
	Open(ctx context.Context) (*Revision, error)
	OpenRevision(ctx context.Context, revision string) (*Revision, error)
}

A Revision holds the templates as an fs.FS, along with the ID to pass back to OpenRevision later. Close removes the fetched files.

NewSource accepts the same remote source forms as Open. For now only git implements it. Any other source returns ErrNoRevisions.

For git:

• The revision is the commit hash, read with git rev-parse HEAD before .git is removed.
• Open keeps the source's ref and depth.
• OpenRevision replaces ref with getit's new commit parameter and sets depth=1, so only that one commit is fetched. This needs git 2.49 or later.

Open is unchanged. getit is bumped to the commit that adds commit (block/getit#4). It also brings fuller git error messages and process-group cancellation for git clones.

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