clone: save hardlink state of util.copyfiles()

When trying to do hardlink-cloning, the os_link() call of the
first file tried already fails on Windows, if the source is on a
UNC path.

This change avoids calling os_link() for the rest of files, leaving
us with a *single* failed os_link() call per clone operation, if the
source can't do hardlinks.

mercurial/hg.py

@@ -277,6 +277,7 b' def clone(ui, source, dest=None, pull=Fa'
                                      % dest)
                 raise
 
+            hardlink = None
             for f in src_repo.store.copylist():
                 src = os.path.join(src_repo.sharedpath, f)
                 dst = os.path.join(dest_path, f)
@@ -287,7 +288,7 b' def clone(ui, source, dest=None, pull=Fa'
                     if dst.endswith('data'):
                         # lock to avoid premature writing to the target
                         dest_lock = lock.lock(os.path.join(dstbase, "lock"))
-                    util.copyfiles(src, dst)
+                    hardlink = util.copyfiles(src, dst, hardlink)
 
             # we need to re-init the repo after manually copying the data
             # into it