analysis.graph: Fix getHead() to return the earliest vertex

analysis.graph: Fix getHead() to return the earliest vertex

Instead of returning the first vertex of the first element, make sure it
returns the earliest vertex.

Change-Id: I4c29fa02f97128ca80e8510560c8ae5512f3d72c
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/66418
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>