cmd/tailscale: let 'tailscale up --reset' do a pref edit

The --reset shouldn't imply that a Backend.Start is necessary.  With
this, it can do a Backend.EditPrefs instead, which then doesn't do all
the heavy work that Start does. Also, Start on Windows behaves
slightly differently than Linux etc in some cases because of tailscaled
running in client mode on Windows (where the GUI supplies the prefs).

Fixes #3702

Change-Id: I75c9f08d5e0052bf623074030a3a7fcaa677abf6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -786,6 +786,16 @@ func TestUpdatePrefs(t *testing.T) {
 			wantSimpleUp:   true,
 			wantJustEditMP: &ipn.MaskedPrefs{WantRunningSet: true},
 		},
+		{
+			name:  "just_edit_reset",
+			flags: []string{"--reset"},
+			curPrefs: &ipn.Prefs{
+				ControlURL: ipn.DefaultControlURL,
+				Persist:    &persist.Persist{LoginName: "crawshaw.github"},
+			},
+			env:            upCheckEnv{backendState: "Running"},
+			wantJustEditMP: &ipn.MaskedPrefs{WantRunningSet: true},
+		},
 		{
 			name:  "control_synonym",
 			flags: []string{},

cmd/tailscale/cli/up.go

@@ -413,7 +413,6 @@ func updatePrefs(prefs, curPrefs *ipn.Prefs, env upCheckEnv) (simpleUp bool, jus
 
 	justEdit := env.backendState == ipn.Running.String() &&
 		!env.upArgs.forceReauth &&
-		!env.upArgs.reset &&
 		env.upArgs.authKeyOrFile == "" &&
 		!controlURLChanged &&
 		!tagsChanged