etc/committer: Record minimal context for hunks to avoid problems.

etc/committer: Record minimal context for hunks to avoid problems.

With zero context new definitions would be applied to the wrong location in
the file.  More context lines lead to larger hunks, though, so we use just one
line of context.

* etc/committer.scm.in (diff-info): Invoke "git diff" with one line of
context.
[info]: Merge line break and first line.
(lines-to-first-change): New procedure.
(old-sexp, new-sexp): Use it.