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.

.gitignore

@@ -1,4 +1,3 @@
-
 /*.cache
 /*.flc
 /*.prj
@@ -7,6 +6,7 @@
 /.version
 /ABOUT-NLS
 /COPYING
+/ChangeLog
 /GNUmakefile
 /INSTALL
 /Makefile
@@ -31,4 +31,3 @@
 /patches
 /releases
 /stamp-h*
-