Git-Support: Extended custom menu. Bugfixes.

- Fixed opening of Git buffers in the presence of tab pages.
- Fixed ':GitBlame' menu entries.
- ':GitBlame' is more conservative when deciding for/against a range.
- Added tags '<FILE>' and '<WORD>' for custom menus.
- Added custom menu to 'additions.vimrc' and 'customization.vimrc'
- Documentation: New options for custom menus.
- Documentation: Added known issue concerning ':GitBlame'.