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>

merge-file.c

@@ -21,7 +21,7 @@ static const char *write_temp_file(mmfile_t *f)
 	fd = mkstemp(filename);
 	if (fd < 0)
 		return NULL;
-	filename = strdup(filename);
+	filename = xstrdup(filename);
 	if (f->size != xwrite(fd, f->ptr, f->size)) {
 		rm_temp_file(filename);
 		return NULL;