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
echo '% hg debuginstall'
hg debuginstall
echo '% hg debuginstall with no username'
HGUSER= hg debuginstall
# Happy End
true