sftp: Add option to disable remote hash check command execution

docs/content/sftp.md

@@ -151,6 +151,10 @@ Modified times are used in syncing and are fully supported.
 
 SFTP supports checksums if the same login has shell access and `md5sum`
 or `sha1sum` as well as `echo` are in the remote's PATH.
+This remote check can be disabled by setting the configuration option
+`disable_hashcheck`. This may be required if you're connecting to SFTP servers
+which are not under your control, and to which the execution of remote commands
+is prohibited.
 
 The only ssh agent supported under Windows is Putty's pageant.
 

sftp/sftp.go

@@ -70,6 +70,10 @@ func init() {
 					Help:  "Enables the use of the aes128-cbc cipher.",
 				},
 			},
+		}, {
+			Name:     "disable_hashcheck",
+			Help:     "Disable the exectution of SSH commands to determine if remote file hashing is available, leave blank unless you know what you are doing.",
+			Optional: true,
 		}},
 	}
 	fs.Register(fsi)
@@ -613,6 +617,11 @@ func (f *Fs) Hashes() fs.HashSet {
 		return *f.cachedHashes
 	}
 
+	hashcheckDisabled := fs.ConfigFileGetBool(f.name, "disable_hashcheck")
+	if hashcheckDisabled {
+		return fs.HashSet(fs.HashNone)
+	}
+
 	c, err := f.getSftpConnection()
 	if err != nil {
 		fs.Errorf(f, "Couldn't get SSH connection to figure out Hashes: %v", err)