feat: Enable setting default --merge-base-with values

* feat: You can now specify a default value for the  parameter in the .lintrunner.toml file. To do so, add a line to the top in the form .  Generally you'll set that to either  or , depending on how your repo is setup.

* Switch to option

* Doc updates

* fix

* lint fixes

.lintrunner.toml

@@ -1,3 +1,5 @@
+merge_base_with="main"
+
 [[linter]]
 code = 'RUSTFMT'
 include_patterns = ['**/*.rs']

README.md

@@ -82,6 +82,8 @@ file, conventionally named `.lintrunner.toml`.
 Here is an example linter configuration:
 
 ```toml
+merge_base_with = 'main'
+
 [[linter]]
 name = 'FLAKE8'
 include_patterns = [

src/lint_config.rs

@@ -10,6 +10,11 @@ use serde::{Deserialize, Serialize};
 pub struct LintRunnerConfig {
     #[serde(rename = "linter")]
     pub linters: Vec<LintConfig>,
+
+    /// The default value for the `merge_base_with` parameter.
+    /// Recommend setting this is set to your default branch, e.g. `main`
+    #[serde()]
+    pub merge_base_with: Option<String>,
 }
 
 fn is_false(b: &bool) -> bool {

src/main.rs

@@ -212,6 +212,13 @@ fn do_main() -> Result<i32> {
         RevisionOpt::Revision(revision)
     } else if let Some(merge_base_with) = args.merge_base_with {
         RevisionOpt::MergeBaseWith(merge_base_with)
+    } else if lint_runner_config.merge_base_with.is_some() {
+        RevisionOpt::MergeBaseWith(
+            lint_runner_config
+                .merge_base_with
+                .clone()
+                .expect("Merge base should be defined"),
+        )
     } else {
         RevisionOpt::Head
     };

tests/integration_test.rs

@@ -47,6 +47,7 @@ fn temp_config_returning_msg(lint_message: LintMessage) -> Result<tempfile::Name
     let serialized = serde_json::to_string(&lint_message)?;
     let config = temp_config(&format!(
         "\
+            merge_base = \"some_base\"
             [[linter]]
             code = 'TESTLINTER'
             include_patterns = ['**']