git-compat-util: add xstrdup_or_null helper

It's a common idiom to duplicate a string if it is non-NULL,
or pass a literal NULL through. This is already a one-liner
in C, but you do have to repeat the name of the string
twice. So if there's a function call, you must write:

  const char *x = some_fun(...);
  return x ? xstrdup(x) : NULL;

instead of (with this patch) just:

  return xstrdup_or_null(some_fun(...));

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

git-compat-util.h

@@ -629,6 +629,11 @@ extern char *xgetcwd(void);
 
 #define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), (alloc) * sizeof(*(x)))
 
+static inline char *xstrdup_or_null(const char *str)
+{
+	return str ? xstrdup(str) : NULL;
+}
+
 static inline size_t xsize_t(off_t len)
 {
 	if (len > (size_t) len)