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/editor/src/element.rs

@@ -3360,8 +3360,7 @@ fn render_blame_entry(
 
     let relative_timestamp = blame_entry_relative_timestamp(&blame_entry, cx);
 
-    let pretty_commit_id = format!("{}", blame_entry.sha);
-    let short_commit_id = pretty_commit_id.chars().take(6).collect::<String>();
+    let short_commit_id = blame_entry.sha.display_short();
 
     let author_name = blame_entry.author.as_deref().unwrap_or("<no name>");
     let name = util::truncate_and_trailoff(author_name, 20);