cleanup: use internal memory allocation wrapper functions everywhere

The "x"-prefixed versions of strdup, malloc, etc. will check whether the
allocation was successful and terminate the process otherwise.

A few uses of malloc were left alone since they already implemented a
graceful path of failure or were in a quasi external library like xdiff.

Additionally, the call to malloc in compat/win32/syslog.c was not modified
since the syslog() implemented there is a die handler and a call to the
x-wrappers within a die handler could result in recursion should memory
allocation fail.  This will have to be addressed separately.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -533,7 +533,7 @@ static void bootstrap_attr_stack(void)
 
 		if (!is_bare_repository() || direction == GIT_ATTR_INDEX) {
 			elem = read_attr(GITATTRIBUTES_FILE, 1);
-			elem->origin = strdup("");
+			elem->origin = xstrdup("");
 			elem->prev = attr_stack;
 			attr_stack = elem;
 			debug_push(elem);