Fix SFTPFileSystem.makedirs to properly handle relative paths

[sasano8]

In SFTPFileSystem, there is a difference in the behavior of mkdir and mkdirs.
mkdirs converts relative paths to absolute paths.

The code to reproduce is below.

import fsspec

fs = fsspec.filesystem(
    "ssh",
    host="localhost",
    port=9200,
    username="root",
    password="pass"
)

fs.mkdir("test_dir1")  # /root/test_dir1
fs.mkdirs("test_dir2")  # /test_dir2

I've pushed code that addresses this issue.

[martindurant]

I don't think I really realised we supported relative paths at all :)

Thanks for this.