ArchiveCommand: Do not warn for unsupported file mode

ArchiveCommand: Do not warn for unsupported file mode

When ArchiveCommand is invoked directly on the command line, these
warnings to stderr

	warning: mode of path/to/some/submodule ignored

are a useful hint, but in the more usual case where an archive is
being served by a server, the intended audience for that message
cannot see stderr.

Later it might be useful to accept a callback to return these warnings
out of band.

Change-Id: I22e79be69859176d85594031d67c6cb3371c4bd2