Stat() is supposed to return valid information for existing
objects and existing prefixes as well.
The current mechanism issues a listing request and searches
for the object or prefix in the returned list, but this could take
time if there are a lot of entries under a parent prefix.
The new commit changes the internal behavior but not the
functional output. It issues a HEAD request first, if there
is no object then assumes it is a prefix and does listing
in order to find it.
As unix cp command complains when you copy a directory into itself,
(such as `cp -r ~/Desktop/ ~/Desktop/sub/`), mc will follow the
same behavior with this commit.
The main reason is that copy or mirror command can be unpredictable
with the style of copy.
Example:
```
$ mc ls -r myminio/testbucket/
[2018-09-25 19:28:17 CET] 8B dir/object
[2018-09-25 19:28:40 CET] 8B object
$ mc -q cp -r myminio/testbucket/ myminio/testbucket/dir/
`myminio/testbucket/dir/object` -> `myminio/testbucket/dir/dir/object`
`myminio/testbucket/object` -> `myminio/testbucket/dir/object`
```
We can notice that `myminio/testbucket/dir/object` is both source and target
in the copy operation. The last copy operation that will occur will rule the
result of the total copy operation and `myminio/testbucket/dir/dir/object`
will have a different content in each copy operation.
Due to the unpredictability mentionned above, we will disallow copying or
mirroring a folder into itself.
Currently even if the command supports encrypted objects,
the syntax verification would fail on them in certain
cases. Since syntax validation of source/targets doesn't
use the parsed encrypt key pairs. This PR fixes this
behavior.
