cocci: FLEX_ALLOC_MEM to FLEX_ALLOC_STR

Ensure that a FLEX_MALLOC_MEM that uses 'strlen' for its 'len' uses
FLEX_ALLOC_STR instead, since these are equivalent forms.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/flex_alloc.cocci

@@ -0,0 +1,13 @@
+@@
+expression str;
+identifier x, flexname;
+@@
+- FLEX_ALLOC_MEM(x, flexname, str, strlen(str));
++ FLEX_ALLOC_STR(x, flexname, str);
+
+@@
+expression str;
+identifier x, ptrname;
+@@
+- FLEXPTR_ALLOC_MEM(x, ptrname, str, strlen(str));
++ FLEXPTR_ALLOC_STR(x, ptrname, str);