submodule: port submodule subcommand 'status' from shell to C

submodule: port submodule subcommand 'status' from shell to C

This aims to make git-submodule 'status' a built-in. Hence, the function
cmd_status() is ported from shell to C. This is done by introducing
four functions: module_status(), submodule_status_cb(),
submodule_status() and print_status().

The function module_status() acts as the front-end of the subcommand.
It parses subcommand's options and then calls the function
module_list_compute() for computing the list of submodules. Then
this functions calls for_each_listed_submodule() looping through the
list obtained.

Then for_each_listed_submodule() calls submodule_status_cb() for each of
the submodule in its list. The function submodule_status_cb() calls
submodule_status() after passing appropriate arguments to the funciton.
Function submodule_status() is responsible for generating the status
each submodule it is called for, and then calls print_status().

Finally, the function print_status() handles the printing of submodule's
status.

Function set_name_rev() is also ported from git-submodule to the
submodule--helper builtin function compute_rev_name(), which now
generates the value of the revision name as required.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>