Improve tab completion in command mode

Pressing tab when all suggestions start with the same substring
will insert that substring (this is how bash autocompletion works).

cmd/micro/autocomplete.go

@@ -49,6 +49,10 @@ func FileComplete(input string) (string, []string) {
 		} else {
 			chosen = suggestions[0]
 		}
+	} else {
+		if len(dirs) > 1 {
+			chosen = strings.Join(dirs[:len(dirs)-1], "/") + "/"
+		}
 	}
 
 	return chosen, suggestions

cmd/micro/messenger.go

@@ -195,6 +195,10 @@ func (m *Messenger) Prompt(prompt, historyType string, completionTypes ...Comple
 					chosen, suggestions = OptionComplete(currentArg)
 				}
 
+				if len(suggestions) > 1 {
+					chosen = chosen + CommonSubstring(suggestions...)
+				}
+
 				if chosen != "" {
 					if len(args) > 1 {
 						chosen = " " + chosen

cmd/micro/util.go

@@ -182,6 +182,34 @@ func runePos(p int, str string) int {
 	return utf8.RuneCountInString(str[:p])
 }
 
+func lcs(a, b string) string {
+	arunes := []rune(a)
+	brunes := []rune(b)
+
+	lcs := ""
+	for i, r := range arunes {
+		if i >= len(brunes) {
+			break
+		}
+		if r == brunes[i] {
+			lcs += string(r)
+		} else {
+			break
+		}
+	}
+	return lcs
+}
+
+func CommonSubstring(arr ...string) string {
+	commonStr := arr[0]
+
+	for _, str := range arr[1:] {
+		commonStr = lcs(commonStr, str)
+	}
+
+	return commonStr
+}
+
 // Abs is a simple absolute value function for ints
 func Abs(n int) int {
 	if n < 0 {

cmd/micro/util_test.go

@@ -1,6 +1,9 @@
 package main
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func TestNumOccurences(t *testing.T) {
 	var tests = []struct {