Provide `phragment.getstate` and `phragment.getpatch` Conduit methods

Summary:
This provides a `phragment.getstate` and a `phragment.getpatch` Conduit method.

`phragment.getstate` - This returns the current state of the fragment and all of it's children.

`phragment.getpatch` - This accepts a base path and a mapping of paths to hashes.  The mapping is for the caller to specify the current state of the files it has.  This returns a list of patches that the caller needs to apply to it's files to get to the latest version.

Test Plan:
Ran the following script in a folder which had content matching a fragment and it's children:

```
#!/bin/bash

STATE=""
for i in $(find ./ -type f); do
    HASH=$(cat $i | sha1sum | awk '{ print $1 }')
    BASE=${i:2}
    STATE="$STATE,\"$BASE\":\"$HASH\""
done
STATE=${STATE:1}
STATE="{$STATE}"

echo '{"path":"tychaia3.zip","state":'$STATE'}' | arc --conduit-uri=http://phabricator.local/ call-conduit phragment.getpatch
```

and I got:

```
{"error":null,"errorMessage":null,"response":[]}
```

I updated one of the child fragments with a new file and ran the script again (patch has been omitted due to it's size):

```
{"error":null,"errorMessage":null,"response":[{"path":"Content\/TitleFont.xnb","hash_old":"4a927d7b90582e50cdd330de9f4b59b0cc5eb5c7","hash_new":"25867504642a3a403102274c68fbb9b430c1980f","patch":"..."}]}
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, staticshock

Maniphest Tasks: T4205

Differential Revision: https://secure.phabricator.com/D7739