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.
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24