Careful when using chdir inside a dirEntries loop

[Andrej Mitrovic]

Here's a little buggy code which most often throws an exception:

    foreach (string entry; dirEntries(curdir, SpanMode.shallow))
    {
        if (entry.isdir)
        {
            foreach (string subentry; dirEntries(entry, SpanMode.shallow))
            {
                if (subentry.isdir)
                {
                    chdir(rel2abs(subentry));
                }
            }
        }
    }

Usually throws something like:
std.file.FileException@std\file.d(1124): .\foo\bar.d: The system cannot find 
the path specified.

The problem is, chdir will modify curdir and dirEntries doesn't store 'curdir' 
internally as an absolute address.

The way to work around this is to call rel2abs as soon as possible when using 
'curdir' (the first line changed):

    foreach (string entry; dirEntries(rel2abs(curdir), SpanMode.shallow))
    {
        if (entry.isdir)
        {
            foreach (string subentry; dirEntries(entry, SpanMode.shallow))
            {
                if (subentry.isdir)
                {
                    chdir(rel2abs(subentry));
                }
            }
        }
    }

I've had a use for code like this because where I was invoking a batch file 
which was always relative to the current subdirectory. But anyways, I just 
thought I'd throw it out in case anyone else runs into this.