- Allows to show staged hunks with different signs.

- Under the hood gitsigns can now run 2 diffs and display signs for
  each:
    - The main sign configuration is config.signs and the sign
    configuration for the second set of signs if config.signs_sec.
    - Secondary set of signs can use any diff base, defaults to HEAD.

- Remove undo_stage_hunk():
    Instead you can just run stage_hunk over a staged hunk.