grep: fix builds with with no thread support

grep: fix builds with with no thread support

Commit 0281e487fd91 ("grep: optionally recurse into submodules")
added functions grep_submodule() and grep_submodule_launch() which
use "struct work_item" which is defined only when thread support
is available.

The original implementation of grep_submodule() used the "struct
work_item" in order to gain access to a strbuf to store its output which
was to be printed at a later point in time.  This differs from how both
grep_file() and grep_sha1() handle their output.  This patch eliminates
the reliance on the "struct work_item" and instead opts to use the
output function stored in the output field of the "struct grep_opt"
object directly, making it behave similarly to both grep_file() and
grep_sha1().

Reported-by: Rahul Bedarkar <redacted>
Signed-off-by: Brandon Williams <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>