submodule: rename helper functions to avoid ambiguity

submodule: rename helper functions to avoid ambiguity

The helper functions: show_submodule_summary(),
prepare_submodule_summary() and print_submodule_summary() are used by
the builtin_diff() function in diff.c to generate a summary of
submodules in the context of a diff. Functions with similar names are to
be introduced in the upcoming port of submodule's summary subcommand.

So, rename the helper functions to '*_diff_submodule_summary()' to avoid
ambiguity.

Mentored-by: Christian Couder <redacted>
Mentored-by: Kaartic Sivaraam <redacted>
Signed-off-by: Shourya Shukla <redacted>
Signed-off-by: Junio C Hamano <redacted>