That way, 'make change-history' needs to change only ChangeLog.2,
instead of having to change two files.
* ChangeLog.2: Add commit info for range that this file covers.
* Makefile.in (new_commit_regexp): New macro.
(change-history-nocommit): Simplify, by putting what used to be
the gen_origin value into the data (ChangeLog.2) rather than
into the program (gitlog-to-emacslog).
* build-aux/gitlog-to-emacslog (gen_origin): Calculate from
the input file (e.g., ChangeLog.2) rather than by having a
constant in the program. Substitute it into the output.