rrs: fix unique constraint on RecipeMaintainerHistory sha1 field

Although it's unlikely to be an issue, technically we shouldn't be
insisting the sha1 field be unique globally, just within each
layerbranch, so adjust the constraints.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

rrs/migrations/0018_rmh_unique.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.12 on 2018-04-18 05:42
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('layerindex', '0013_patch'),
+        ('rrs', '0017_maintenanceplan_maintainer_style'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='recipemaintainerhistory',
+            name='sha1',
+            field=models.CharField(max_length=64),
+        ),
+        migrations.AlterUniqueTogether(
+            name='recipemaintainerhistory',
+            unique_together=set([('layerbranch', 'sha1')]),
+        ),
+    ]

rrs/models.py

@@ -225,9 +225,12 @@ class RecipeMaintainerHistory(models.Model):
     title = models.CharField(max_length=255, blank=True)
     date = models.DateTimeField(db_index=True)
     author = models.ForeignKey(Maintainer)
-    sha1 = models.CharField(max_length=64, unique=True)
+    sha1 = models.CharField(max_length=64)
     layerbranch = models.ForeignKey(LayerBranch)
 
+    class Meta:
+        unique_together = ('layerbranch', 'sha1',)
+
     @staticmethod
     def get_last(layerbranch):
         rmh_qry = RecipeMaintainerHistory.objects.filter(layerbranch=layerbranch).order_by('-date')