submodule.h: use a named enum for RECURSE_SUBMODULES_*

submodule.h: use a named enum for RECURSE_SUBMODULES_*

Using a named enum allows casting an integer to the enum type in both
GDB and LLDB:

    $ gdb -q -ex 'b wt-status.c:44' -ex r --args ./git status
    (gdb) p (enum color_wt_status) slot
    $1 = WT_STATUS_ONBRANCH

    $ lldb -o 'b wt-status.c:44' -o r -- ./git status
    (lldb) p (color_wt_status) slot
    (color_wt_status) $0 = WT_STATUS_ONBRANCH

In LLDB, it's also required to cast in the reversed direction, i.e.
cast an enum constant into its corresponding integer:

    (lldb) p (int) color_wt_status::WT_STATUS_ONBRANCH
    (int) $1 = 8

Name the enum listing the different RECURSE_SUBMODULES_* modes, to make
debugging easier. For example, when stepping through a part of the code
where an int is compared with a constant in this enum, it allows casting
the int to the enum type or vice-versa, after quickly checking where the
enum constant is declared and learning the enum name.

As to not make this patch a debug-only change, convert the
'fetch_recurse' member of 'struct submodule' to use the newly named
enum.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>