manifest: Only support -o option on XML formatted manifest

If the manifest isn't a single file format manifest, the -o option
makes no sense, as you cannot export multiple files to a single
stream for display or redirection.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/manifest.py

@@ -17,6 +17,7 @@ import os
 import sys
 
 from command import PagedCommand
+from manifest_xml import XmlManifest
 
 def _doc(name):
   r = os.path.dirname(__file__)
@@ -31,7 +32,7 @@ class Manifest(PagedCommand):
   common = False
   helpSummary = "Manifest inspection utility"
   helpUsage = """
-%prog [-o {-|NAME.xml} [-r]]
+%prog [options]
 """
   _xmlHelp = """
 
@@ -50,6 +51,7 @@ in a Git repository for use during future 'repo init' invocations.
     return help
 
   def _Options(self, p):
+    if isinstance(self.manifest, XmlManifest):
       p.add_option('-r', '--revision-as-HEAD',
                    dest='peg_rev', action='store_true',
                    help='Save revisions as current HEAD')
@@ -73,7 +75,8 @@ in a Git repository for use during future 'repo init' invocations.
     if args:
       self.Usage()
 
-    if opt.output_file is not None:
+    if  isinstance(self.manifest, XmlManifest) \
+    and opt.output_file is not None:
         self._Output(opt)
         return