fsmonitor: allow core.fsmonitor = "none" to disable

When doing things like testing snapshot performance differences,
this allows you to turn off the monitor, no matter what the enabled
user or repository configuration has, e.g.

    jj st --config-toml='core.fsmonitor="none"'

Signed-off-by: Austin Seipp <[email protected]>

lib/src/fsmonitor.rs

@@ -34,6 +34,13 @@ pub enum FsmonitorKind {
         /// reporting.
         changed_files: Vec<PathBuf>,
     },
+
+    /// No filesystem monitor. This is the default if nothing is configured, but
+    /// also makes it possible to turn off the monitor on a case-by-case basis
+    /// when the user gives an option like
+    /// `--config-toml='core.fsmonitor="none"'`; useful when e.g. when doing
+    /// analysis of snapshot performance.
+    None,
 }
 
 impl FromStr for FsmonitorKind {
@@ -45,6 +52,7 @@ impl FromStr for FsmonitorKind {
             "test" => Err(config::ConfigError::Message(
                 "cannot use test fsmonitor in real repository".to_string(),
             )),
+            "none" => Ok(Self::None),
             other => Err(config::ConfigError::Message(format!(
                 "unknown fsmonitor kind: {}",
                 other

lib/src/local_working_copy.rs

@@ -1021,6 +1021,7 @@ impl TreeState {
     ) -> Result<FsmonitorMatcher, SnapshotError> {
         let (watchman_clock, changed_files) = match fsmonitor_kind {
             None => (None, None),
+            Some(FsmonitorKind::None) => (None, None),
             Some(FsmonitorKind::Test { changed_files }) => (None, Some(changed_files)),
             Some(FsmonitorKind::Watchman) => {
                 if cfg!(feature = "watchman") {