Run GIT-VERSION-GEN with $(SHELL), not sh.

Alas, not all shells named sh are capable enough to run
GIT-VERSION-GEN.
Signed-off-by: NJason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -67,7 +67,7 @@ all:
 # change being considered an inode change from the update-cache perspective.
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
-	@sh ./GIT-VERSION-GEN
+	@$(SHELL) ./GIT-VERSION-GEN
 -include GIT-VERSION-FILE
 
 # CFLAGS and LDFLAGS are for the users to override from the command line.