diffmanifests: honor user-supplied manifest paths

The current implementation ignores the user-specified paths to
manifests.  if the "repo diffmanifests" is invoked with absolute
file paths for one or both manifests, the command fails with message:

fatal: duplicate path ... in /tmp/manifest-old.xml

Also the current implementation fails to expand the absolute path to
manifest files if "repo diffmanifests" is invoked with relative
paths, i.e "repo diffmanifests manifest-old.xml manifest-new.xml".

fatal: manifest manifest-old.xml not found

This commit fixes the first issue by disabling the local manifest
discovery for diffmanifests command, and the second issue by
expanding paths to manifests within "diffmanifests" sub-command.

Test: repo manifest --revision-as-HEAD -o /tmp/manifest-old.xml
      repo sync
      repo manifest --revision-as-HEAD -o /tmp/manifest-new.xml
      repo diffmanifests /tmp/manifest-old.xml /tmp/manifest-new.xml
Change-Id: Ia125d769bfbea75adb9aba81abbd8c636f2168d4
Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>
diff --git a/subcmds/diffmanifests.py b/subcmds/diffmanifests.py
index 751a202..a1e823c 100644
--- a/subcmds/diffmanifests.py
+++ b/subcmds/diffmanifests.py
@@ -190,12 +190,12 @@
       self.printProject = self.printAdded = self.printRemoved = self.printRevision = self.printText
 
     manifest1 = XmlManifest(self.manifest.repodir)
-    manifest1.Override(args[0])
+    manifest1.Override(args[0], load_local_manifests=False)
     if len(args) == 1:
       manifest2 = self.manifest
     else:
       manifest2 = XmlManifest(self.manifest.repodir)
-      manifest2.Override(args[1])
+      manifest2.Override(args[1], load_local_manifests=False)
 
     diff = manifest1.projectsDiff(manifest2)
     if opt.raw: