signing: allow disabling by setting backend to "none"

We don't seem to have a way to override the config to disable
signing. This patch lets you do that by setting
`signing.backend="none"`.

cli/src/config-schema.json

@@ -473,7 +473,8 @@
                 "backend": {
                     "type": "string",
                     "enum": ["gpg", "ssh"],
-                    "description": "The backend to use for signing commits"
+                    "description": "The backend to use for signing commits. The string `none` disables signing.",
+                    "default": "none"
                 },
                 "key": {
                     "type": "string",

docs/config.md

@@ -776,6 +776,8 @@ GnuPG or SSH signing keys.
 
 To do this you need to configure a signing backend.
 
+Setting the backend to `"none"` disables signing.
+
 ### GnuPG Signing
 
 ```toml

lib/src/settings.rs

@@ -245,7 +245,8 @@ impl UserSettings {
     // separate from sign_settings as those two are needed in pretty different
     // places
     pub fn signing_backend(&self) -> Option<String> {
-        self.config.get_string("signing.backend").ok()
+        let backend = self.config.get_string("signing.backend").ok()?;
+        (backend.as_str() != "none").then_some(backend)
     }
 
     pub fn sign_settings(&self) -> SignSettings {