git-gui: fix error popup when doing blame -> "Show History Context"

In d9c6469 (git-gui: update status bar to track operations, 2019-12-01) the call to 'ui_status' in 'do_gitk' was updated to create the newly introduced "status bar operation". This allowed this status text to show along with other operations happening in parallel, and removed a race between all these operations. But in that refactor, the fact that 'ui_status' checks for the existence of 'main_status' was overlooked. This leads to an error message popping up when the user selects "Show History Context" from the blame window context menu on a source line. The error occurs because when running "blame" 'main_status' is not initialized. So, add a check for the existence of 'main_status' in 'do_gitk'. This fix reverts to the original behaviour. In the future, we might want to look into a better way of telling 'do_gitk' which status bar to use. Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>