git-instaweb: respect bindir from Makefile

Not everybody installs git to /usr/bin/git.
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -575,7 +575,7 @@ git-instaweb: git-instaweb.sh gitweb/gitweb.cgi gitweb/gitweb.css
 	    -e '/@@GITWEB_CGI@@/d' \
 	    -e '/@@GITWEB_CSS@@/r gitweb/gitweb.css' \
 	    -e '/@@GITWEB_CSS@@/d' \
-	    $@.sh > $@+
+	    $@.sh | sed "s|/usr/bin/git|$(bindir)/git|" > $@+
 	chmod +x $@+
 	mv $@+ $@