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.
# -- setting up base repo
adding a
# -- cloning base repo
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
# -- setting up cset to push
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
adding a
created new head
# -- pushing
pushing to ../a
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files (+1 heads)