Bugfix: Adding published packages to aptly repo truncates the files

Hi @smira 

Check out this weird behaviour of aptly:

On a ubuntu precise box, I have installed aptly version 0.8.
I have run the following commands:

```
root@packages:~# aptly repo create main

Local repo [main] successfully added.
You can run 'aptly repo add main ...' to add packages to repository.
```
```
root@packages:~# aptly mirror create sensu http://repos.sensuapp.org/apt sensu main
...
Mirror [sensu]: http://repos.sensuapp.org/apt/ sensu successfully added.
You can run 'aptly mirror update sensu' to download repository contents.
```
```
root@packages:~# aptly mirror update sensu
...
Mirror `sensu` has been successfully updated.
```
```
root@packages:~# aptly snapshot create sensu from mirror sensu

Snapshot sensu successfully created.
You can run 'aptly publish snapshot sensu' to publish snapshot as Debian repository.
```
```
root@packages:~# aptly publish snapshot sensu
Loading packages...
...
You can also use `aptly serve` to publish your repositories over HTTP quickly.
```

```
root@packages:~# cd /var/lib/aptly/public/pool/main/s/sensu
root@packages:~# ls -l
total 1020148
-rw-r--r-- 2 root root 35217340 Oct 16 12:56 sensu_0.10.0-1_amd64.deb
-rw-r--r-- 2 root root 35219162 Oct 16 12:58 sensu_0.10.0-2_amd64.deb
...
-rw-r--r-- 2 root root 15921398 Oct 16 12:57 sensu_0.9.8-1_amd64.deb
-rw-r--r-- 2 root root 15920958 Oct 16 12:58 sensu_0.9.9-1_amd64.deb
```
At this point, I wish to add all these packages to the main repo:
```
root@packages:/var/lib/aptly/public/pool/main/s/sensu# aptly repo add main .
Loading packages...
[+] sensu_0.10.0-1_amd64 added
[+] sensu_0.10.0-2_amd64 added
...
[+] sensu_0.9.8-1_amd64 added
[+] sensu_0.9.9-1_amd64 added
```
At this point, another `ls -l` reveals that the files in the directory are now empty:
```
root@packages:/var/lib/aptly/public/pool/main/s/sensu# ls -l
total 0
-rw-r--r-- 2 root root 0 Oct 16 15:25 sensu_0.10.0-1_amd64.deb
-rw-r--r-- 2 root root 0 Oct 16 15:25 sensu_0.10.0-2_amd64.deb
...
-rw-r--r-- 2 root root 0 Oct 16 15:25 sensu_0.9.8-1_amd64.deb
-rw-r--r-- 2 root root 0 Oct 16 15:25 sensu_0.9.9-1_amd64.deb
```

After some investigation, we've come to the conclusion that the files are effectively hard links to files in `/var/lib/aptly/pool`. A copy truncates the destination file - which is also the source - before copying the file across. This is the reason why all the added files appear empty.

This PR addresses this issue, simply skipping the copy of the files when the source and destination are hard linked.

I've also tested the changes to make sure the resulting behaviour is indeed expected.

I've also added a test to verify this bug won't appear anymore. The test will be run on travis with this PR.