commit.c: Remove backward goto in read_craft_line()

commit.c: Remove backward goto in read_craft_line()

Bad graft data is noticed in several places in read_graft_line(), and in
each case we go back to the first site of detection.  It in general is a
better style to have an exception handling out of line from the main
codepath and make error codepath jump forward.

Signed-off-by: Ralf Thielow <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>