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.

build-aux/.gitignore

@@ -1,4 +1,3 @@
-
 /announce-gen
 /arg-nonnull.h
 /c++defs.h
@@ -9,6 +8,7 @@
 /depcomp
 /gendocs.sh
 /git-version-gen
+/gitlog-to-changelog
 /install-sh
 /javacomp.sh.in
 /javaexec.sh.in
@@ -21,4 +21,3 @@
 /vc-list-files
 /warn-on-use.h
 /ylwrap
-

build-aux/git-log-fix

@@ -0,0 +1,3 @@
+# This file is expected to be used via gitlog-to-changelog's --amend=FILE
+# option.  It specifies what changes to make to each given SHA1's commit
+# log and metadata, using Perl-eval'able expressions.