Fix contrib/subtree Makefile to patch #! line

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -18,6 +18,11 @@ RM       ?= rm -f
 ASCIIDOC = asciidoc
 XMLTO    = xmlto
 
+ifndef SHELL_PATH
+	SHELL_PATH = /bin/sh
+endif
+SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
+
 ASCIIDOC_CONF = ../../Documentation/asciidoc.conf
 MANPAGE_XSL   = ../../Documentation/manpage-normal.xsl
 
@@ -32,7 +37,8 @@ GIT_SUBTREE_HTML := git-subtree.html
 all: $(GIT_SUBTREE)
 
 $(GIT_SUBTREE): $(GIT_SUBTREE_SH)
-	cp $< $@ && chmod +x $@
+	sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' $< >$@
+	chmod +x $@
 
 doc: $(GIT_SUBTREE_DOC) $(GIT_SUBTREE_HTML)