DepthGenerator: fix multi-child boundary handling

DepthGenerator: fix multi-child boundary handling

Suppose that a repository has the following commit graph:

 B   C
  \ /
   A

and it was cloned with --shallow-exclude=A. DepthGenerator does not mark
C as shallow, causing an invalid repository to be produced on the
client, because A is not sent. (A similar issue occurs when
--shallow-since is used to exclude A but neither B nor C.)

This happens whenever an excluded commit has more than one child that is
to be sent to the client. Fix DepthGenerator to handle this case
correctly.

While we're editing DepthWalk.Commit, fix the documentation of
DepthWalk.Commit#isBoundary.

Change-Id: I7068abf0fe0c864d1b0e56e1616dad1aa8719411
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>