git-gui: Update Russian translation

Also, the previous translations of the words 'tag' and 'merge' were
changed. Added translation of the 'Tool' submenu.

Thanks go to Alexander Gavrilov and Dmitry Potapov for proofreading
and suggestions.
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>