VERSION CONTROL WRAPPER

Each version control module must provide these functions:

XXX_command(url_tail, url_dict, nodelist, action)
		Execute a version control system command.
XXX_list(url_tail, url_dict, directory, recursive)
		List entries currently in the version control system.

Where "XXX" is equal to what appears in the "commit" attribute before "://".
E.g., for "sccs://" the function is sccs_command().


Common arguments are:

url_tail	String: part of the "commit" attribute after XXX://.
		For: {commit = cvs://:pserver:user@foo.org/root {path = bar}}
		url_tail is: :pserver:user@foo.org/root
		It can be empty, in which case the module must figure out the
		required parameters itself.

url_dict	Dictionary of the "commit" attribute and its attributes.
		For: {commit = cvs://asdf/root {path = foobar}}
		url_dict is: {"name" : "cvs://asdf/root", "path" : "foobar"}.
		Useful attributes:
			path		path to directory with nodes
			logentry	message for commit log
			tag		name for this version


The arguments for XXX_command() are:

nodelist	List of Node objects for which the action is to be carried
		out.

action		String: Command to be executed.  First argument of the
		":verscont" command.  Predefined are:

	refresh		Update local version from repository.
			No-op for files that are already up-to-date.

	commit		Update the repository for local changes.
			No-op for file that didn't change.
			File is added to repository when necessary.
			Do checkout/checkin when checkout is required.
			Don't change locking of the file.

	publish		Like commit and leave the file unlocked.  May also add
			a tag to this version.

	checkout	Like refresh and additionally lock for editing
			when possible.

	checkin		Like commit and leave the file unlocked.

	unlock		Remove lock on file, don't change file in repository
			or locally.

	add		Add file to repository.  File does exist locally.

	remove		Remove file from repository.  File may exist locally.


The return value must be zero when something went wrong, non-zero when the
command worked successfully.

The function can throw a UserError exception when called with wrong arguments.

When there is something wrong that prevents version control to work (e.g., the
repository doesn't exist) it should silently returning zero.  A-A-P may try
another method to obtain the file.


The arguments for XXX_list() are:

directory	Absolute path name of the directory for which the listing is
		requested.

recursive	When non-empty or non-zero: recursively list directories.

The return value must be a list of full file names.  Directories are also to
be included.