Monkey patch SSHKit to expose current backend

Eventually this will be part of SSHKit itself, but for flexibility in using
Capistrano with older SSHKit versions, I'm including this here as a monkey
patch.

The point of SSHKit::Backend.current is to give code access to the current
Backend without having to pass around a reference to it. For example, consider
a `git` helper that executes a git command:

def git(*args)
  SSHKit::Backend.current.execute(:git, *args)
end

Thanks to `current`, we can use this git method wherever we are in an `on`
block, without having to pass a reference to `self`:

on release_roles(:all) do
  git "status"
end

Without the thread local, the same task would be much more awkward due to the
necessary passing of `self`:

def git(backend, *args)
  backend.execute(:git, *args)
end

on release_roles(:all) do
  git self, "status"
end

To someone not intimately familiar with SSHKit, what `self` in this context is
confusing and not obvious at all. It is because SSHKit uses `instance_exec`
within the `on` block that `self` magically transforms.

Better to avoid using `self` altogether and prefer the thread local.

lib/capistrano/ext/sshkit/backend/thread_local.rb

@@ -0,0 +1,25 @@
+require "sshkit/backends/abstract"
+
+# Monkey patch older versions of SSHKit to make the currently-executing Backend
+# available via a thread local value.
+#
+# TODO: Remove this code once capistrano.gemspec requires newer SSHKit version.
+
+unless SSHKit::Backend.respond_to?(:current)
+  module SSHKit
+    module Backend
+      def self.current
+        Thread.current["sshkit_backend"]
+      end
+
+      class Abstract
+        def run
+          Thread.current["sshkit_backend"] = self
+          instance_exec(@host, &@block)
+        ensure
+          Thread.current["sshkit_backend"] = nil
+        end
+      end
+    end
+  end
+end

spec/lib/capistrano/ext/sshkit/backend/thread_local_spec.rb

@@ -0,0 +1,30 @@
+require "spec_helper"
+require "capistrano/ext/sshkit/backend/thread_local"
+
+module SSHKit
+  module Backend
+    describe "#current" do
+      require "sshkit/dsl"
+
+      it "refers to the currently executing backend" do
+        backend = nil
+        current = nil
+
+        on(:local) do
+          backend = self
+          current = SSHKit::Backend.current
+        end
+
+        expect(current).to eq(backend)
+      end
+
+      it "is nil outside of an on block" do
+        on(:local) do
+          # nothing
+        end
+
+        expect(SSHKit::Backend.current).to be_nil
+      end
+    end
+  end
+end