Git: Rename 'Undo uncommitted repository changes' to 'Reset...'.

'Reset' is in sync with the command line option and more
appropriate to the functionality after the introduction
of the history dialog.

Change-Id: I2328c7137089a5f372b59b4db7074753a16ecbdf
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@nokia.com>