maint: generate ChangeLog from git log

* Makefile.am (gen-ChangeLog): New rule.
(dist-hook): Depend on it.
(EXTRA_DIST): Distribute the two ChangeLog-* files.
* bootstrap.conf (gnulib_modules): Add gitlog-to-changelog.
(bootstrap_post_import_hook): Ensure that ChangeLog exists.
* build-aux/git-log-fix: New file.
* ChangeLog-2012: Renamed ...
* ChangeLog: ... from this.
* ChangeLog-1998: Renamed ...
* OChangeLog: ...from this
* .gitignore: Add ChangeLog.

Makefile.am

@@ -27,8 +27,7 @@ aclocaldir = @aclocaldir@
 aclocal_DATA = m4/bison-i18n.m4
 
 EXTRA_DIST = .prev-version .version \
-  cfg.mk \
-  OChangeLog PACKAGING
+  cfg.mk ChangeLog-1998 ChangeLog-2012 PACKAGING
 
 # Initialization before completion by local.mk's.
 AM_CFLAGS = $(WARN_CFLAGS)
@@ -62,7 +61,7 @@ include tests/local.mk
 BUILT_SOURCES += $(top_srcdir)/.version
 $(top_srcdir)/.version: configure
 	echo $(VERSION) > $@-t && mv $@-t $@
-dist-hook:
+dist-hook: gen-ChangeLog
 	echo $(VERSION) > $(distdir)/.tarball-version
 
 .PHONY: update-b4-copyright update-package-copyright-year
@@ -73,3 +72,15 @@ update-b4-copyright:
 	@echo 'warning: src/parse-gram.[hc] may need to be regenerated.'
 update-package-copyright-year:
 	$(build_aux)/$@ configure.ac
+
+gen_start_date = 2012-01-16
+.PHONY: gen-ChangeLog
+gen-ChangeLog:
+	$(AM_V_GEN)if test -d .git; then				\
+	  $(top_srcdir)/build-aux/gitlog-to-changelog			\
+	    --no-cluster						\
+	    --amend=$(srcdir)/build-aux/git-log-fix			\
+	    --since=$(gen_start_date) > $(distdir)/cl-t;		\
+	  rm -f $(distdir)/ChangeLog;					\
+	  mv $(distdir)/cl-t $(distdir)/ChangeLog;			\
+	fi