Git-Support: New actions for various buffers.

- Added new actions to the branch, remote, and status buffer.
- Improved action 'reset' in the status buffer.
Git-Support: Improved help and show commands.

- ':GitHelp' shows man page text with appropriate width.
- ':GitShow' recognizes :0/1/2/3:<file> and names the buffer appropriately.
- ':GitCommitFile' updates buffer before committing.
- Internal changes.