filter-branch: use $SHELL_PATH instead of 'sh'

On some systems, 'sh' isn't very friendly. In particular,
t7003 fails on Solaris because it doesn't understand $().
Instead, use the specified SHELL_PATH to run shell code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -873,6 +873,7 @@ common-cmds.h: $(wildcard Documentation/git-*.txt)
 $(patsubst %.sh,%,$(SCRIPT_SH)) : % : %.sh
 	$(QUIET_GEN)$(RM) $@ $@+ && \
 	sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
+	    -e 's|@SHELL_PATH@|$(SHELL_PATH_SQ)|' \
 	    -e 's|@@PERL@@|$(PERL_PATH_SQ)|g' \
 	    -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
 	    -e 's/@@NO_CURL@@/$(NO_CURL)/g' \

git-filter-branch.sh

@@ -309,7 +309,7 @@ while read commit parents; do
 	sed -e '1,/^$/d' <../commit | \
 		eval "$filter_msg" > ../message ||
 			die "msg filter failed: $filter_msg"
-	sh -c "$filter_commit" "git commit-tree" \
+	@SHELL_PATH@ -c "$filter_commit" "git commit-tree" \
 		$(git write-tree) $parentstr < ../message > ../map/$commit
 done <../revs