Add -p to repo forall to improve output formatting

When trying to read log output from many projects at once it can
be difficult to make sense of which messages came from where.

For many professional developers it is common to want to view the
last week's worth of your work, so you can write a weekly summary
of your activity for your status report.

This is easier with the new -p option:

  repo forall -pc git log --reverse --since=1.week.ago --author=sop

produces a report of all commits written by me in the last week,
formatted in a paged output display, with headers inserted in
front of each project's output.

Where this can be even more useful is with git log's pickaxe,
e.g. now we can use:

  repo forall -pc git log -Sbar v1.0..v1.1

to locate all additions or removals of the symbol 'bar' since v1.0,
up to and including v1.1.  Before displaying the matching commits in
a project, a project header is shown, giving the user some context
information for the matching results.

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

command.py

@@ -27,6 +27,9 @@ class Command(object):
   manifest = None
   _optparse = None
 
+  def WantPager(self, opt):
+    return False
+
   @property
   def OptionParser(self):
     if self._optparse is None:
@@ -109,11 +112,15 @@ class InteractiveCommand(Command):
   """Command which requires user interaction on the tty and
      must not run within a pager, even if the user asks to.
   """
+  def WantPager(self, opt):
+    return False
 
 class PagedCommand(Command):
   """Command which defaults to output in a pager, as its
      display tends to be larger than one screen full.
   """
+  def WantPager(self, opt):
+    return True
 
 class MirrorSafeCommand(object):
   """Command permits itself to run within a mirror,