Replace icon for merge commit

The new icon now shows a merge graph rather than a '!'.

Users might find the '!' scary and may think that they have done
something totally wrong.

The merge icon is based on the important icon from the Tango Icon
Library [1]

[1] http://tango.freedesktop.org/Tango_Icon_Library

Change-Id: I59904b894df9074444df8a7810f1f224a7e2f6aa
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
Merge "Replace icon for merge commit" into stable-2.9