Refresh git log buffer with `r`

[BertrandSim]

In a GV buffer, hit r to refresh the git log print out.

This achieves the same goal as (#53), but uses a different implementation.

In particular:

• Hitting r will call s:refresh().
• s:refresh() first empties the current GV buffer, then s:fill()s it with the same git_log_cmd
• git_log_cmd is changed to be a buffer-local variable (b:), so that one can reuse the same git log command for the same buffer.

The third point enables s:refresh() to be used with multiple GV instances. In addition, other split windows (such as the diff window) are also preserved in the current tab.

[BertrandSim]

rebased on latest master

[somini]

I merged this with #48 on my fork (https://github.com/somini/gv.vim). This stores the command on the existing metadata dict at b:gv.