cmd/dist: implement fetch prototype

With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

progress/bar.go

@@ -0,0 +1,65 @@
+package progress
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// TODO(stevvooe): We may want to support more interesting parameterization of
+// the bar. For now, it is very simple.
+
+// Bar provides a very simple progress bar implementation.
+//
+// Use with fmt.Printf and "r" to format the progress bar. A "-" flag makes it
+// progress from right to left.
+type Bar float64
+
+var _ fmt.Formatter = Bar(1.0)
+
+func (h Bar) Format(state fmt.State, r rune) {
+	switch r {
+	case 'r':
+	default:
+		panic(fmt.Sprintf("%v: unexpected format character", float64(h)))
+	}
+
+	if h > 1.0 {
+		h = 1.0
+	}
+
+	if h < 0.0 {
+		h = 0.0
+	}
+
+	if state.Flag('-') {
+		h = 1.0 - h
+	}
+
+	width, ok := state.Width()
+	if !ok {
+		// default width of 40
+		width = 40
+	}
+
+	var pad int
+
+	extra := len([]byte(green)) + len([]byte(reset))
+
+	p := make([]byte, width+extra)
+	p[0], p[len(p)-1] = '|', '|'
+	pad += 2
+
+	positive := int(Bar(width-pad) * h)
+	negative := width - pad - positive
+
+	n := 1
+	n += copy(p[n:], []byte(green))
+	n += copy(p[n:], bytes.Repeat([]byte("+"), positive))
+	n += copy(p[n:], []byte(reset))
+
+	if negative > 0 {
+		n += copy(p[n:len(p)-1], bytes.Repeat([]byte("-"), negative))
+	}
+
+	state.Write(p)
+}