Allow snapshot to be created from empty local repo

Fixes #288

system/t05_snapshot/CreateSnapshot9Test_gold

@@ -1 +1,3 @@
-ERROR: unable to create snapshot: local repo doesn't have packages
+
+Snapshot snap9 successfully created.
+You can run 'aptly publish snapshot snap9' to publish snapshot as Debian repository.

system/t05_snapshot/create.py

@@ -106,4 +106,3 @@ class CreateSnapshot9Test(BaseTest):
         "aptly repo create local-repo",
     ]
     runCmd = "aptly snapshot create snap9 from repo local-repo"
-    expectedCode = 1

system/t12_api/publish.py

@@ -105,9 +105,6 @@ class PublishSnapshotAPITest(APITest):
         snapshot_name = self.random_name()
         self.check_equal(self.post("/api/repos", json={"Name": repo_name}).status_code, 201)
 
-        resp = self.post("/api/repos/" + repo_name + '/snapshots', json={'Name': snapshot_name})
-        self.check_equal(resp.status_code, 400)
-
         d = self.random_name()
         self.check_equal(self.upload("/api/files/" + d,
                          "libboost-program-options-dev_1.49.0.1_i386.deb").status_code, 200)

system/t12_api/snapshots.py

@@ -97,8 +97,12 @@ class SnapshotsAPITestCreateFromRepo(APITest):
         self.check_equal(self.post("/api/repos", json={"Name": repo_name}).status_code, 201)
 
         resp = self.post("/api/repos/" + repo_name + '/snapshots', json={'Name': snapshot_name})
-        self.check_equal(resp.status_code, 400)
+        self.check_equal(resp.status_code, 201)
+        self.check_equal([],
+                         self.get("/api/snapshots/" + snapshot_name + "/packages", params={"format": "details"}).json())
 
+
+        snapshot_name = self.random_name()
         d = self.random_name()
         self.check_equal(self.upload("/api/files/" + d,
                          "libboost-program-options-dev_1.49.0.1_i386.deb").status_code, 200)