Clean up 'make changelog'

We need more information in the changelog, and we need some checking to
make sure the .git dir exists before trying to generate it.

Makefile.am

@@ -13,11 +13,21 @@ EXTRA_DIST=	ChangeLog configure.ac \
 		tools/gas2masm/gas2masm.dsp tools/gas2masm/gas2masm.dsw \
 		tools/gas2masm/gas2masm.mak tools/gas2masm/gas2masm.mdp
 
-ChangeLog:
-	git log > ChangeLog
-
 NOCONV_DIST=	$(distdir)/include/win32/resources/icon1.ico
 
+changelog: ChangeLog
+ChangeLog: FORCE
+	@if test -d "$(srcdir)/.git"; then \
+		echo "creating ChangeLog" && \
+		(	cd "$(top_srcdir)" && \
+			echo 'This file is generated by Makefile; do not edit.'; \
+			echo; \
+			$(top_srcdir)/missing --run git log --abbrev-commit --stat --no-merges \
+		) > $(top_builddir)/ChangeLog; \
+	else \
+		echo 'A git clone is required to generate ChangeLog' >&2; \
+	fi
+
 dist-zip: distdir
 	-chmod -R a+r $(distdir)
 	ZIP="-r9q" zip $(distdir).zip $(NOCONV_DIST)
@@ -36,3 +46,5 @@ dist-all-local: distdir
 	ZIP="-r9q" zip $(distdir).zip $(NOCONV_DIST)
 	ZIP="-r9ql" zip $(distdir).zip $(distdir) -x $(NOCONV_DIST)
 	-rm -rf $(distdir)
+
+FORCE: