Add `nightly` recipe

Installs the last revision in the previous `RUBY_RELEASE_DATE`.

common.mk

@@ -1887,6 +1887,22 @@ ChangeLog:
 	-e 'VCS.detect(ARGV[0]).export_changelog(path: ARGV[1])' \
 	"$(srcdir)" $@
 
+# CAUTION: If using GNU make 3 which does not support `.WAIT`, this
+# recipe with multiple jobs makes build and `git reset` run
+# simultaneously, and will cause inconsistent results.  Run with `-j1`
+# or update GNU make.
+nightly: yesterday $(DOT_WAIT) install
+	$(NULLCMD)
+
+# Rewind to the last commit "yesterday".  "Yesterday" means here the
+# period where `RUBY_RELEASE_DATE` is the day before the date to be
+# generated now.  In short, the yesterday in JST-9 time zone.
+yesterday: rewindable
+
+rewindable:
+	$(GIT) -C $(srcdir) status --porcelain
+	$(GIT) -C $(srcdir) diff --quiet
+
 HELP_EXTRA_TASKS = ""
 
 help: PHONY

template/Makefile.in

@@ -687,3 +687,7 @@ yes-test-syntax-suggest: $(PREPARE_SYNTAX_SUGGEST)
 		$(RSPECOPTS) spec/syntax_suggest/$(SYNTAX_SUGGEST_SPECS)
 	$(ACTIONS_ENDGROUP)
 no-test-syntax-suggest:
+
+yesterday:
+	$(GIT) -C $(srcdir) reset --hard \
+	    `$(GIT) -C $(srcdir) log -1 --before=00:00+0900 --format=%H`

win32/Makefile.sub

@@ -1438,3 +1438,8 @@ rubyspec-capiext: $(RUBYSPEC_CAPIEXT_EXTS)
 !endif
 
 exts: rubyspec-capiext
+
+yesterday:
+	for /f "usebackq" %H in \
+	    (`$(GIT) -C $(srcdir) log -1 "--before=00:00+0900" "--format=%H"`) do \
+	    $(GIT) -C $(srcdir) reset --hard %%H