Use merge date as a primary date for commit

Merge date is calculated as a date when the corresponding review was merged. Commit date
is shown as separate field.

Resolves bug 1204928

The idea of the fix is proposed by Chandan Kumar (chandankumar-093047)

Change-Id: I7e3f38b3d4cdecf6e888382964df2187ece6a343
Merge "Use merge date as a primary date for commit"