Deploy upon merge

[aik099]

In some workflows after merging a code (e.g. from trunk to a tags/stable tag), it's required to deploy these changes to the server (e.g. production or staging).

This can be implemented as follows:

1. add a per-repository (no global version) config setting called commit.deploy-command, which will store a Bash command that will do a deploy;
2. when a commit is performed (automatic after merge or manual) AND svn:mergeinfo property was changed AND commit.deploy-command config setting is specified, then execute it without buffering so that the user can see results in real-time.

[aik099]

Implemented in #156 .