Don't use 'mark-whole-buffer' for 'stage-whole-buffer'

'mark-whole-buffer' has the unwanted side effect of changing the user's
mark ring.
Add changes in master branch to changelog
Add 'stage-and-commit-whole-buffer', 'unstage-whole-buffer'

Trivial, but superbly helpful in day-to-day git use.
Support the latest 'git-commit-mode'

Don't derive from 'git-commit-mode', but copy the initial setup code and
omit statements concerning async committing.

This fixes a bug where output from a diff subprocess was inserted into the commit
message buffer because `kill-buffer-query-functions' was modified by 'git-commit-mode'.
Fix commit message history cycling

By locally resetting 'log-edit-comment-ring-index',
'M-p' is now guaranteed to insert the previous commit message
Add a comment about newer Magit versions
Refresh when a buffer is staged or unstaged in Magit
Readme: Add animated GIF to show staging/committing
Fix the splitting logic for displaying the staged changes buffer
Version 0.2