Improve ssh handling (#277)

checkout: use configured protocol for PR checkout

instead of defaulting to ssh if that is enabled
this might fix #262

login add: try to find a matching ssh key & store it in config

possibly expensive operation should be done once

pr checkout: don't fetch ssh keys

As a result, we don't try to pull via ssh, if no privkey was configured.
This increases chances of a using ssh only on a working ssh setup.

fix import order

remove debug print statement

improve ssh-key value docs

rm named return & fix pwCallback nil check

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/277
Reviewed-by: khmarbaise <khmarbaise@noreply.gitea.io>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: 6543 <6543@obermui.de>
Co-Authored-By: Norwin <noerw@noreply.gitea.io>
Co-Committed-By: Norwin <noerw@noreply.gitea.io>

cmd/login/add.go

@@ -52,7 +52,7 @@ var CmdLoginAdd = cli.Command{
 		&cli.StringFlag{
 			Name:    "ssh-key",
 			Aliases: []string{"s"},
-			Usage:   "Path to a SSH key to use for pull/push operations",
+			Usage:   "Path to a SSH key to use, overrides auto-discovery",
 		},
 		&cli.BoolFlag{
 			Name:    "insecure",