editor: Push em width calculations down into EditorSnapshot::gutter_dimensions (#24062)

This PR removes the `em_width` and `em_advance` parameters to
`EditorSnapshot::gutter_dimensions` in favor of computing the values
inside of it.

In practice all of the callers were passing in the same values, and
there isn't a circumstance where we would want to pass in different
values.

`gutter_dimensions` has also been modified to return
`Option<GutterDimensions>` instead of `GutterDimensions` so that we can
remove some `.unwrap`s when interacting with the text system.

Release Notes:

- N/A

crates/git/src/git.rs

@@ -45,6 +45,9 @@ actions!(
     ]
 );
 
+/// The length of a Git short SHA.
+pub const SHORT_SHA_LENGTH: usize = 7;
+
 #[derive(Clone, Copy, Eq, Hash, PartialEq)]
 pub struct Oid(libgit::Oid);
 
@@ -64,7 +67,7 @@ impl Oid {
 
     /// Returns this [`Oid`] as a short SHA.
     pub fn display_short(&self) -> String {
-        self.to_string().chars().take(7).collect()
+        self.to_string().chars().take(SHORT_SHA_LENGTH).collect()
     }
 }