Makefile: use SHELL_PATH when running generate-cmdlist.sh

Non-POSIX shells, such as /bin/sh on SunOS, do not support $((...))
arithmetic expansion or $(...) command substitution needed by
generate-cmdlist.sh.  Make sure that we use a POSIX compliant shell
$(SHELL_PATH) when running generate-cmdlist.sh.
Signed-off-by: NAlejandro R. Sedeño <asedeno@mit.edu>
Acked-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1690,7 +1690,7 @@ $(BUILT_INS): git$X
 common-cmds.h: generate-cmdlist.sh command-list.txt
 
 common-cmds.h: $(wildcard Documentation/git-*.txt)
-	$(QUIET_GEN)./generate-cmdlist.sh command-list.txt >$@+ && mv $@+ $@
+	$(QUIET_GEN)$(SHELL_PATH) ./generate-cmdlist.sh command-list.txt >$@+ && mv $@+ $@
 
 SCRIPT_DEFINES = $(SHELL_PATH_SQ):$(DIFF_SQ):$(GIT_VERSION):\
 	$(localedir_SQ):$(NO_CURL):$(USE_GETTEXT_SCHEME):$(SANE_TOOL_PATH_SQ):\