Generate the ChangeLog reproducibly.

* build-aux/gitlog-to-changelog-repro.patch: New file.
* Makefile.am (EXTRA_DIST): Add it.
(build-aux/gitlog-to-changelog-repro): New rule.
(gen-ChangeLog): Use 'gitlog-to-changelog-repro'.
(DISTCLEANFILES): Add 'build-aux/gitlog-to-changelog-repro'.

.gitignore

@@ -9,6 +9,7 @@
 !/build-aux/git-version-gen
 !/build-aux/gitlab-ci.yml
 !/build-aux/gitlog-to-changelog
+!/build-aux/gitlog-to-changelog-repro.patch
 !/build-aux/guile.am
 !/build-aux/pre-inst-env.in
 /config.log

Makefile.am

@@ -27,6 +27,7 @@ EXTRA_DIST +=                                   \
   build-aux/git-version-gen                     \
   build-aux/gitlab-ci.yml                       \
   build-aux/gitlog-to-changelog                 \
+  build-aux/gitlog-to-changelog-repro.patch     \
   guix.scm                                      \
   tests/data/README
 
@@ -206,10 +207,17 @@ dist-hook: gen-ChangeLog
 	    cp $(srcdir)/.tarball-timestamp $(distdir)/.tarball-timestamp; \
 	fi
 
+build-aux/gitlog-to-changelog-repro: \
+		$(srcdir)/build-aux/gitlog-to-changelog \
+		$(srcdir)/build-aux/gitlog-to-changelog-repro.patch
+	$(AM_V_GEN){ mkdir -p $$(dirname $@) && \
+	             patch -t --output=$@ $? && \
+	             chmod a+x $@; }
+
 .PHONY: gen-ChangeLog
-gen-ChangeLog:
+gen-ChangeLog: build-aux/gitlog-to-changelog-repro
 	$(AM_V_GEN)if test -d .git; then \
-	    { $(top_srcdir)/build-aux/gitlog-to-changelog \
+	    { ./build-aux/gitlog-to-changelog-repro \
 	          --format='%s%n%n%b%n' -- v0.1.. && \
 	      echo && \
 	      sed -n -e '/^Copyright/,$$p' < $(top_srcdir)/ChangeLog; \
@@ -250,6 +258,8 @@ CLEANFILES +=                                   \
   $(UNIT_TESTS:tests/%.scm=tests/%.log)         \
   $(UNIT_TESTS:tests/%.scm=tests/%.trs)
 
+DISTCLEANFILES = build-aux/gitlog-to-changelog-repro
+
 clean-local:
 	$(MAKE) $(AM_MAKEFLAGS) -L -C tests/spec clean
 

build-aux/gitlog-to-changelog-repro.patch

@@ -0,0 +1,16 @@
+This patch adjusts the 'gitlog-to-changelog' script from Gnulib so
+that its output is reproducible.
+
+diff --git a/build-aux/gitlog-to-changelog b/build-aux/gitlog-to-changelog
+index deddef2..6fec950 100755
+--- a/build-aux/gitlog-to-changelog
++++ b/build-aux/gitlog-to-changelog
+@@ -345,7 +345,7 @@ sub git_dir_option($)
+                   ? '  (tiny change)' : '');
+ 
+       my $date_line = sprintf "%s  %s$tiny\n",
+-        strftime ("%Y-%m-%d", localtime ($1)), $2;
++        strftime ("%Y-%m-%d", gmtime ($1)), $2;
+ 
+       my @coauthors = grep /^Co-authored-by:.*$/, @line;
+       # Omit meta-data lines we've already interpreted.