transport: create Client interface (#132)

* plumbing: move plumbing/client package to plumbing/transport.

* transport: create Client interface.

* A Client can instantiate any client transport service.
* InstallProtocol installs a Client for a given protocol,
  instead of just a UploadPackService.
* A Client can open a session for fetch-pack or send-pack
  for a specific Endpoint.
* Adapt ssh and http clients to the new client interface.

* updated doc

1 files changed, 32 insertions, 0 deletions

diff --git a/plumbing/transport/client/client.go b/plumbing/transport/client/client.go
new file mode 100644
index 0000000..5c6da05
--- /dev/null
+++ b/plumbing/transport/client/client.go
@@ -0,0 +1,32 @@
+package client
+
+import (
+	"fmt"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/transport"
+	"gopkg.in/src-d/go-git.v4/plumbing/transport/http"
+	"gopkg.in/src-d/go-git.v4/plumbing/transport/ssh"
+)
+
+// Protocols are the protocols supported by default.
+var Protocols = map[string]transport.Client{
+	"http":  http.DefaultClient,
+	"https": http.DefaultClient,
+	"ssh":   ssh.DefaultClient,
+}
+
+// InstallProtocol adds or modifies an existing protocol.
+func InstallProtocol(scheme string, c transport.Client) {
+	Protocols[scheme] = c
+}
+
+// NewClient returns the appropriate client among of the set of known protocols:
+// HTTP, SSH. See `InstallProtocol` to add or modify protocols.
+func NewClient(endpoint transport.Endpoint) (transport.Client, error) {
+	f, ok := Protocols[endpoint.Scheme]
+	if !ok {
+		return nil, fmt.Errorf("unsupported scheme %q", endpoint.Scheme)
+	}
+
+	return f, nil
+}