Make download patchset filename match link on UI

The patchset file name link in the download dialog box has an
abbreviated git sha of 7 characters while the actual downloaded file
contains 8 characters. Set the downloaded filename to 7 chars to
match the link on the UI.

Change-Id: I97bc4a0c4ff4f67b3ad2667fe1176dbae2c493c8
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetPatch.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetPatch.java
index 3c4d79d..a13e7be 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetPatch.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetPatch.java
@@ -167,7 +167,7 @@
 
   private static String fileName(RevWalk rw, RevCommit commit)
       throws IOException {
-    AbbreviatedObjectId id = rw.getObjectReader().abbreviate(commit, 8);
+    AbbreviatedObjectId id = rw.getObjectReader().abbreviate(commit, 7);
     return id.name() + ".diff";
   }
 }