sequencer: factor out parsing of todo commands

sequencer: factor out parsing of todo commands

Move the code that parses todo commands into a separate function so
that it can be shared with "git status" in the next commit. As we
know the input is NUL terminated we do not pass a pointer to the end
of the line and instead test for a blank line by looking for NUL, CR
LF, or LF. We use starts_with() instead of starts_with_mem() for the
same reason. This results in slightly different behavior when there
a CR at the start of the line that is not followed by LF. Previously
such a line was treated as a comment rather than an invalid line.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>