Generate ChangeLog from git log

* .gitignore: Add ChangeLog
* ChangeLog: Rename to ChangeLog-CVS.
* Makefile.am (gen-changelog): New rule.
(dist-hook): Depend on it.
(EXTRA_DIST): Add ChangeLog-CVS.
* README-hacking: Describe changes.
* gitlog-to-changelog: Import from gnulib.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
diff --git a/.gitignore b/.gitignore
index 9616077..2afc8fe 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,8 @@
 .libs
 .*.swp
 
+ChangeLog
+
 Makefile
 Makefile.in