Add `GREP_SOURCE_SUBMODULE` as a grep_source type and cases for this new
type in the various switch statements in grep.c.
When initializing a grep_source with type `GREP_SOURCE_SUBMODULE` the
identifier can either be NULL (to indicate that the working tree will be
used) or a SHA1 (the REV of the submodule to be grep'd). If the
identifier is a SHA1 then we want to fall through to the
`GREP_SOURCE_SHA1` case to handle the copying of the SHA1.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>