update: Use antiquated workflow for refreshing working copy, if safe

Usage:
  eg update

Description:
  Gets updates from the default remote repository if updating is safe, and
  provides suggestions on proceeding otherwise.

  eg update does not accept any options...other than --help.

Examples:
  Get any updates from the remote repository
      $ eg update

Differences from git update:
  eg update is unique to eg; it exists primarily to ease the transition for
  cvs/svn users and to do something useful for them.  In particular, eg
  update is used just to do fast-forward updates when there are no local
  changes; if anything more than this is needed, eg advises users to run
  other commands.

  Here are the special cases eg update detects and provides tailored
  messages for:
    * User has local commits           => ask user to use eg pull instead
    * User provides argument to update => tell user to use eg switch for
                                          checking out an older revision or
                                          eg revert to undo changes to a file
    * User has locally deleted files   => tell user to use eg revert to
                                          undo local changes (and that they do
                                          not need to delete the file first as
                                          they did with cvs)
    * User has local modifications     => Tell user to stash or commit their
                                          changes before pulling updates
    * No default repository to contact => Tell user to run "eg remote add
                                          origin REPOSITORY_URL"
    * branch.BRANCH.merge not set and  => Warn user that we do not know which
      more than one remote branch         branch to pull from and suggest eg
      present                             pull or setting branch.BRANCH.merge

See also
  Run 'git help pull' for a comprehensive list of options available.
  eg update is designed to accept the same options as git pull, and
  with the same meanings unless specified otherwise in the above
  "Differences" section.