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improve kubectl cp command in several ways
**Release note**:
"kubectl cp" process soft link in better ways as well as some little bugs
**Soft link**:
before this patch
"kubectl cp" command will copy the soft link to destination as an empty regular file
after this patch
"kubectl cp" command will behave the same as tar command
this patch improves it on both from container and to container
**some bugs**
1.from container to host
a.when copy a file ends with '/', it will cause a panic.
for example, container gakki has a regular file /tmp/test, then run command
_kubectl cp gakki:/tmp/test/ /tmp_
a panic happens
b.when copy a file which does not exist in container, the command ends up without
any error information
2.from host to container
a.when run command like
kubectl cp "" gakki:/tmp
it will try cp current directory to container, in other words, this command works
the same as kubectl cp . gakki:/tmp
b.current cp command will omit an empty directory
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