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

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>