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.
* etc/committer.scm.in (<hunk>)[diff]: Rename this field...
[diff-lines]: ...to this.
[definition?]: New field.
(hunk->patch): Join diff lines.
(diff-info): Do not join diff lines; record whether a hunk is a new
definition.
(commit-message): Rename this procedure...
(change-commit-message): ...to this.
(add-commit-message): New procedure.
(main): Handle new package definitions before changes.
* etc/committer.scm.in (commit-message): Use EQUAL? instead of EQ? to compute
differences because not all inputs may be symbols; some could be expressions.
