Right-clicking a commit on blame shows the parent commit relevant for this file.

In case the file was renamed, fetching the parent can fail (since the file with
this name did not exist before the requested commit).

Using the direct parent is good enough, and should work for all cases.

What the user is really interested in is "how did the file look before this
commit", and the exact "previous" commit is not really interesting.

Change-Id: I7d74efce5f2b065f62e978ba5f14d3010a5f2c7f
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>