Export template_dir in Makefile.

If somebody set template_dir in config.mak. Then git-init-db would be
compiled with the correct location but the templates would be installed
in the default location. Fix it.
Signed-off-by: NTom Prince <tom.prince@ualberta.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -310,7 +310,7 @@ SCRIPTS = $(patsubst %.sh,%,$(SCRIPT_SH)) \
 	  $(patsubst %.py,%,$(SCRIPT_PYTHON)) \
 	  gitk
 
-export prefix TAR INSTALL DESTDIR SHELL_PATH
+export prefix TAR INSTALL DESTDIR SHELL_PATH template_dir
 ### Build rules
 
 all: $(PROGRAMS) $(SCRIPTS)