Increase short SHA length to 7 characters (#11492)

This PR increases the length of a shortened Git SHA from 6 to 7
characters.

This matches what GitHub uses.

I also took the opportunity to factor out a common method for computing
a short SHA so that we have a single source of truth for the length that
we're using.

Release Notes:

- Increased the short commit SHA length used by git blame from 6 to 7
characters.

crates/git/src/git.rs

@@ -37,6 +37,11 @@ impl Oid {
     pub(crate) fn is_zero(&self) -> bool {
         self.0.is_zero()
     }
+
+    /// Returns this [`Oid`] as a short SHA.
+    pub fn display_short(&self) -> String {
+        self.to_string().chars().take(7).collect()
+    }
 }
 
 impl FromStr for Oid {