The old surrounding-sexp procedure would read all S-expressions from the
beginning of the file up to the given line number and then return the last
encountered S-expression. This is quite wasteful. Instead we can record all
lines that begin with an S-expression and jump straight to the offset closest
to the desired line number to read the S-expression there.
* etc/committer.scm.in (lines+offsets-with-opening-parens): New procedure.
(surrounding-sexp): Use it.