unstage: Mark changes in files as no longer ready for commit

Usage:
  eg unstage [--] PATH...

Description:
  Marks the changes in the specified files as not being ready to commit.
  When a directory is passed, all files in that directory or any
  subdirectory are recursively unstaged.

  Note that this command is equivalent to 'eg revert --staged PATH...'

  See 'eg help topic staging' for more details, including situations where
  you might find staging useful.

Examples:
  Create a new file, and mark it for addition to the repository, then change
  your mind
      $ echo hi > there
      $ eg stage there
      $ eg unstage there

  Modify an existing file, mark the modified version as being ready for commit,
  then change your mind
      $ echo some extra info at end of file >> foo
      $ eg stage foo
      $ eg unstage foo

Differences from git unstage:
  eg unstage is a command new to eg that is not part of git; it is implemented
  on top of eg revert --staged, though it could as easily simply call through
  to git reset.