Add support for Registry condition

[steffen-wilke]

In order to allow for a more flexible configuration, it would be nice to have a RegistryCondition.
This would enable temporary behavior depending on a current registry entry on the server system. It could be used while a backup is in process or a branch is created.

Example:

<FailInstruction Message="MyPath is currently locked.">
  <Conditions Type="And">
    <RegistryCondition Key="HKEY_CURRENT_USER\SOFTWARE\scm\MyPathIsLocked" Value="1" /> 
    <ChangedPathCondition ChangedPathRegExPattern="/mypath/.*" />
  </Conditions>
</FailInstruction>

[BrightLight]

To be honest, I'm not seeing any benefits to have such a condition. A RegistryCondition would require to have access to the Subversion server machine to change the registry. It seems you want to have some sort of "flag" option to change behavior (in #13 you mentioned that you want to use environment variables for similar reasons).
But if you have to log onto the machine anyway (to change the registry), why not directly change the RepoCop configuration to modify behavior? For instance, where I work we sometimes need to block commits to certain parts of the repository while we create a new version of our product. To do this, we have commented blocks in our RepoCop configuration which I uncomment during the initial creation of the new version and later comment again.
One suggestion to make it more convenient to temporarily change behavior would be to introduce some sort of variables or flags directly in the configuration file (maybe a "ConstantCondition" in combination with named conditions for re-use). It would allow to put some distance between critical parts of the configuration and these flags. But to be honest, until now this wasn't really necessary. I do it in Notepad++ and I simply do an XML validation before I save.
One more thing why I'm against the registry thing: I'm toying with the idea to migrate this tool to .Net Core (which should be possible, now that it no longer depends on SharpSvn, thanks to your PR).

[steffen-wilke]

Well, I still see some value in having a configuration-external flag because it would be easier to automate setting the flag e.g. via PowerShell without having to adjust the XML configuration.
A script to modify the XML and automate the current state would be more complicated to achieve.

I totally understand the argument to move this library to .NET Core though.
Accessing the registry would require Windows Compatibility Pack and would not be beneficial for other operating systems.