checklink: work around sshfs brain-damage (issue3636)

With the follow_symlinks option, sshfs will successfully create links
while claiming it encountered an I/O error. In addition, depending on
the type of link, it may subsequently be impossible to delete the link
via sshfs. Our existing link to '.' will cause sshfs to think the link
is a directory, and thus cause unlink to give EISDIR. Links to
non-existent names or circular links will cause the created link to not even
be visible.

Thus, we need to create a new temporary file and link to that. We'll
still get a failure, but we'll be able to remove the link.

mercurial/posix.py

@@ -154,10 +154,16 b' def checklink(path):'
     # file already exists
     name = tempfile.mktemp(dir=path, prefix='hg-checklink-')
     try:
-        os.symlink(".", name)
+        fd = tempfile.NamedTemporaryFile(dir=path, prefix='hg-checklink-')
+        os.symlink(os.path.basename(fd.name), name)
         os.unlink(name)
         return True
-    except (OSError, AttributeError):
+    except AttributeError:
+        return False
+    except OSError, inst:
+        # sshfs might report failure while successfully creating the link
+        if inst[0] == errno.EIO and os.path.exists(name):
+            os.unlink(name)
         return False
 
 def checkosfilename(path):