mk: Add make target "repro" for easy bug testing

* mk/default.mk (REPRO_DEBUG): New custom variable enabling extra
debugging facilities in Emacs instance open by make repro.
(REPRO_ARGS): Extra arguments passed to Emacs by make repro.
(REPRO_INIT):
(REPRO): Define Emacs cmd for testing.
* mk/targets.mk (.PHONY):
(repro): Add new target.
* doc/org-manual.org (Feedback): Describe the new make target.

doc/org-manual.org

@@ -269,6 +269,14 @@ shown below.
 (add-to-list 'load-path (expand-file-name "/path/to/org-mode/lisp"))
 #+end_src
 
+If you are using Org mode version from Git repository, you can start
+minimal session using make.
+
+: # Bare Emacs
+: make repro
+: # or pass extra arguments
+: make repro REPRO_ARGS="-l /path/to/minimal/config.el /tmp/bug.org"
+
 If an error occurs, a "backtrace" can be very useful---see below on
 how to create one.  Often a small example file helps, along with clear
 information about:

mk/default.mk

@@ -59,6 +59,11 @@ BTEST_OB_LANGUAGES = awk C fortran maxima lilypond octave perl python
 # extra packages to require for testing
 BTEST_EXTRA =
               # ess-site  # load ESS for R tests
+# Whether to activate extra debugging facilities for make repro.
+REPRO_DEBUG ?= yes
+# Extra arguments passed to Emacs for make repro.
+# e.g. -l config.el /tmp/bug.org
+REPRO_ARGS ?=
 ##->8-------------------------------------------------------------------
 ## YOU MAY NEED TO ADAPT THESE DEFINITIONS
 ##----------------------------------------------------------------------
@@ -93,6 +98,22 @@ BTEST = $(BATCH) $(BTEST_INIT) \
 # should be useful for manual testing and verification of problems.
 NOBATCH = $(EMACSQ) $(BTEST_INIT) -l org -f org-version
 
+ifeq ($(REPRO_DEBUG), yes)
+REPRO_INIT = --eval "(setq \
+	debug-on-error t\
+	debug-on-signal nil\
+	debug-on-quit nil\
+	org-element--cache-self-verify 'backtrace\
+	org-element--cache-self-verify-frequency 1.0\
+	org-element--cache-map-statistics t)"
+else
+REPRO_INIT =
+endif
+
+# Running a plain emacs with no config, this Org mode loaded, and
+# debugging facilities activated.
+REPRO = $(NOBATCH) $(REPRO_INIT) $(REPRO_ARGS)
+
 # start Emacs with no user and site configuration
 # EMACSQ = -vanilla # XEmacs
 EMACSQ  = $(EMACS)  -Q

mk/targets.mk

@@ -41,7 +41,7 @@ endif
 	cleanlisp cleandoc cleandocs cleantest \
 	compile compile-dirty uncompiled \
 	config config-test config-exe config-all config-eol config-version \
-	vanilla
+	vanilla repro
 
 CONF_BASE = EMACS DESTDIR ORGCM ORG_MAKE_DOC
 CONF_DEST = lispdir infodir datadir testdir
@@ -132,6 +132,9 @@ $(INSTSUB):
 autoloads: lisp
 	$(MAKE) -C $< $@
 
+repro: cleanall autoloads
+	-@$(REPRO) &
+
 cleandirs:
 	$(foreach dir, $(SUBDIRS), $(MAKE) -C $(dir) cleanall;)