Documentation/Makefile: make doc.dep dependencies a variable again

Re-introduce a variable to declare what *.txt files need to be
considered for the purposes of scouring files to generate a dependency
graph of includes.

When doc.dep was introduced in a5ae8e64cf (Fix documentation
dependency generation., 2005-11-07) we had such a variable called
TEXTFILES, but it was refactored away just a few commits after that in
fb612d54c1 (Documentation: fix dependency generation.,
2005-11-07). I'm planning to add more wildcards here, so let's bring
it back.

I'm not calling it TEXTFILES because we e.g. don't consider
Documentation/technical/*.txt when generating the graph (they don't
use includes). Let's instead call it DOC_DEP_TXT.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -3,6 +3,7 @@ MAN1_TXT =
 MAN5_TXT =
 MAN7_TXT =
 HOWTO_TXT =
+DOC_DEP_TXT =
 TECH_DOCS =
 ARTICLES =
 SP_ARTICLES =
@@ -45,6 +46,9 @@ MAN7_TXT += gitworkflows.txt
 
 HOWTO_TXT += $(wildcard howto/*.txt)
 
+DOC_DEP_TXT += $(wildcard *.txt)
+DOC_DEP_TXT += $(wildcard config/*.txt)
+
 ifdef MAN_FILTER
 MAN_TXT = $(filter $(MAN_FILTER),$(MAN1_TXT) $(MAN5_TXT) $(MAN7_TXT))
 else
@@ -288,7 +292,7 @@ docdep_prereqs = \
 	mergetools-list.made $(mergetools_txt) \
 	cmd-list.made $(cmds_txt)
 
-doc.dep : $(docdep_prereqs) $(wildcard *.txt) $(wildcard config/*.txt) build-docdep.perl
+doc.dep : $(docdep_prereqs) $(DOC_DEP_TXT) build-docdep.perl
 	$(QUIET_GEN)$(RM) $@+ $@ && \
 	$(PERL_PATH) ./build-docdep.perl >$@+ $(QUIET_STDERR) && \
 	mv $@+ $@