From: Philippe Blain <levraiphilippeblain@gmail.com>

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

    (gdb) p (enum diff_submodule_format) options->submodule_format
    $1 = DIFF_SUBMODULE_LOG

    (lldb) p (diff_submodule_format) options->submodule_format
    (diff_submodule_format) $1 = DIFF_SUBMODULE_LOG

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) diff_submodule_format::DIFF_SUBMODULE_SHORT
    (int) $0 = 0

Name the enum listing the different RECURSE_SUBMODULES_* modes, to make
debugging easier.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1111.git.1641410782015.gitgitgadget@gmail.com