util.copyfiles: don't try os_link() again if it failed before
If the os_link() call on the first file in the directory fails [1],
we switch mode to using shutil.copy() for all remaining files.
[1] happens for example on Windows for every file when cloning from a UNC
path without specifying --pull.
#!/bin/sh
"$TESTDIR/hghave" symlink || exit 80
hg init a
ln -s a link
cd a
echo foo > foo
hg status
hg status ../link