Documentation: robustify asciidoc GIT_VERSION replacement

Instead of using sed on the resulting file, we now have a
git_version asciidoc attribute. This means that we don't
pipe the output of asciidoc, which means we can detect build
failures.

Problem reported by Scott Lamb, solution suggested by Jonas Fonseca.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/Makefile

@@ -112,8 +112,7 @@ clean:
 %.html : %.txt
 	rm -f $@+ $@
 	$(ASCIIDOC) -b xhtml11 -d manpage -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -o - $< | \
-		sed -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' >$@+
+		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $<
 	mv $@+ $@
 
 %.1 %.5 %.7 : %.xml
@@ -122,8 +121,7 @@ clean:
 %.xml : %.txt
 	rm -f $@+ $@
 	$(ASCIIDOC) -b docbook -d manpage -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -o - $< | \
-		sed -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' >$@+
+		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $<
 	mv $@+ $@
 
 user-manual.xml: user-manual.txt user-manual.conf

Documentation/asciidoc.conf

@@ -40,7 +40,7 @@ template::[header-declarations]
 <refentrytitle>{mantitle}</refentrytitle>
 <manvolnum>{manvolnum}</manvolnum>
 <refmiscinfo class="source">Git</refmiscinfo>
-<refmiscinfo class="version">@@GIT_VERSION@@</refmiscinfo>
+<refmiscinfo class="version">{git_version}</refmiscinfo>
 <refmiscinfo class="manual">Git Manual</refmiscinfo>
 </refmeta>
 <refnamediv>