VCS: Abort previous command for the same editor

It is very common to have output from a previous (longer) command appear
instead of the newly created one.

For example: Open a log. Check "Show Diff" and immediately uncheck it.
If this is done fast enough (or with a high Log Count limit) the output
with the diff will appear instead of the normal log.

Change-Id: Ie64cc4a383a261f5328a4d0486eec93f73766679
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>