Git: Fix crash on Grep

If the process is already finished, and cancel is triggered Creator crashes.

For example, search for "q" in qt-creator, wait for >200K popup, then wait a few
seconds for the process to finish and press Cancel.

The reason for the crash is that reportFinished() is called twice - one from
GitGrepRunner::finish() and another one from Utils::runAsyncImpl().

Change-Id: I61d379f5e3a5ae86c9a48a3751dbb2e00203516d
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>