Replace uses of strdup with xstrdup.

Like xmalloc and xrealloc xstrdup dies with a useful message if
the native strdup() implementation returns NULL rather than a
valid pointer.

I just tried to use xstrdup in new code and found it to be missing.
However I expected it to be present as xmalloc and xrealloc are
already commonly used throughout the code.

[jc: removed the part that deals with last_XXX, which I am
 finding more and more dubious these days.]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

http-fetch.c

@@ -787,7 +787,7 @@ static int remote_ls(struct alt_base *repo, const char *path, int flags,
 
 	ls.flags = flags;
 	ls.repo = repo;
-	ls.path = strdup(path);
+	ls.path = xstrdup(path);
 	ls.dentry_name = NULL;
 	ls.dentry_flags = 0;
 	ls.userData = userData;