git-submodule - Add 'foreach' subcommand

git-submodule - Add 'foreach' subcommand

submodule foreach <command-list> will execute the list of commands in
each currently checked out submodule directory. The list of commands
is arbitrary as long as it is acceptable to sh. The variables '$path'
and '$sha1' are availble to the command-list, defining the submodule
path relative to the superproject and the submodules's commitID as
recorded in the superproject (this may be different than HEAD in the
submodule).

This utility is inspired by a number of threads on the mailing list
looking for ways to better integrate submodules in a tree and work
with them as a unit. This could include fetching a new branch in each
from a given source, or possibly checking out a given named branch in
each. Currently, there is no consensus as to what additional commands
should be implemented in the porcelain, requiring all users whose needs
exceed that of git-submodule to do their own scripting. The foreach
command is intended to support such scripting, and in particular does
no error checking and produces no output, thus allowing end users
complete control over any information printed out and over what
constitutes an error. The processing does terminate if the command-list
returns an error, but processing can easily be forced for all
submodules be terminating the list with ';true'.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>