Fix bug with PoolPath field being overwritten on mirror update

While updating mirror, if package file is already in pool path,
field `PoolPath` was left as empty which results in package file
being unavailable later on while publishing.

system/t06_publish/PublishSnapshot37Test_gold

@@ -0,0 +1,13 @@
+Loading packages...
+Generating metadata files and linking package files...
+Finalizing metadata files...
+Signing file 'Release' with gpg, please enter your passphrase when prompted:
+Clearsigning file 'Release' with gpg, please enter your passphrase when prompted:
+
+Snapshot wheezy has been successfully published.
+Please setup your webserver to serve directory '${HOME}/.aptly/public' with autoindexing.
+Now you can add following line to apt sources:
+  deb http://your-server/ wheezy main
+Don't forget to add your GPG key to apt with apt-key.
+
+You can also use `aptly serve` to publish your repositories over HTTP quickly.

system/t06_publish/snapshot.py

@@ -1001,3 +1001,18 @@ class PublishSnapshot36Test(BaseTest):
         self.check_not_exists('public/dists/maverick/main/Contents-i386.gz')
         self.check_exists('public/dists/maverick/main/binary-amd64/Release')
         self.check_not_exists('public/dists/maverick/main/Contents-amd64.gz')
+
+
+class PublishSnapshot37Test(BaseTest):
+    """
+    publish snapshot: mirror with double mirror update
+    """
+    fixtureGpg = True
+    fixtureCmds = [
+        "aptly -architectures=i386,amd64 mirror create -keyring=aptlytest.gpg -filter='$$Source (gnupg)' -with-udebs wheezy http://mirror.yandex.ru/debian/ wheezy main non-free",
+        "aptly mirror update -keyring=aptlytest.gpg wheezy",
+        "aptly mirror update -keyring=aptlytest.gpg wheezy",
+        "aptly snapshot create wheezy from mirror wheezy",
+    ]
+    runCmd = "aptly publish snapshot -keyring=${files}/aptly.pub -secret-keyring=${files}/aptly.sec wheezy"
+    gold_processor = BaseTest.expand_environ