submodule: add die_in_unpopulated_submodule function
Currently 'git add' is the only command which dies when launched from an
unpopulated submodule (the place-holder directory for a submodule which
hasn't been checked out). This is triggered implicitly by passing the
PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE flag to 'parse_pathspec()'.
Instead make this desire more explicit by creating a function
'die_in_unpopulated_submodule()' which dies if the provided 'prefix' has
a leading path component which matches a submodule in the the index.
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>