revert: make commit subjects in insn sheet optional

revert: make commit subjects in insn sheet optional

Change the instruction sheet format subtly so that the subject of the
commit message that follows the object name is optional.  As a result,
an instruction sheet like this is now perfectly valid:

  pick 35b0426
  pick fbd5bbcbc2e
  pick 7362160f

While at it, also fix a bug introduced by 5a5d80f4 (revert: Introduce
--continue to continue the operation, 2011-08-04) that failed to read
lines that are too long to fit on the commit-id-shaped buffer we
currently use; eliminate the need for the buffer altogether.  In
addition to literal SHA-1 hexes, you can now safely use expressions
like the following in the instruction sheet:

  featurebranch~4
  rr/revert-cherry-pick-continue^2~12@{12 days ago}

[jc: simplify parsing]

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>