`babel update` should not rewrite the catalog's comment header

[lelit]

Currently when babel updates the .po files it rewrites the catalog's header, with the result that some bits of information is thrown away: that header contains both a copyright and first author, and both may be precious data that should not be clobbered. Some project may prefer keeping the whole series of copyright years (i.e. Copyright © 1999, 2005-2015 by me) instead of just the current one, for example.
#82 proposes to make it customizable: I think that would be nice, when babel initializes a new catalog, but maybe it should not touch the header at all, when the catalog gets updated, task that is better left to the developer, for example with the aid of Emacs' copyright.el feature.

Alternatively, babel update should let me say something like “keep the last N lines of the previous header comment and re-add them as-is”.

[lelit]

I put the following snippet in my makefiles as a quick&dirty workaround:

POs := $(wildcard $(LOCALEDIR)/*/LC_MESSAGES/*-client.po)

define fix-first-author =
for po in $(POs); do \
  sed -i -e "s/^# FIRST AUTHOR <EMAIL@ADDRESS>,.*$$/# `git log --date=format:%Y --pretty=format:'%cN <%cE>, %ad.' $$po | tail -1`/" $$po; \
done
endef

.PHONY: update-catalogs
update-catalogs:
    ...
    $(fix-first-author)