git_ui: Count commit title length by characters instead of bytes (#57025)

# What

- Replace `title.len()` with `title.chars().count()` in the commit title
length check, so the limit is measured in Unicode characters instead of
UTF-8 bytes.
- Applied in two places that share the same logic:
  - `crates/git_ui/src/git_panel.rs` — Git panel's inline warning
  - `crates/git_ui/src/commit_modal.rs` — commit modal's inline warning

# Why

The commit title length check used str::len(), which returns UTF-8 byte
length rather than character count. As a result, titles containing
multi-byte characters (Japanese, Chinese, emoji, etc.) triggered the
warning far below the configured commit_title_max_length — around 24
characters instead of the default 72.


Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
  - Before


https://github.com/user-attachments/assets/a895530c-2f73-470c-97fa-29d9467c14e1

  - After


https://github.com/user-attachments/assets/ffbe1ba2-0ccc-4b02-87f5-836da7841dd9


- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- Fixed commit title length check miscounting multi-byte characters as
multiple characters.

crates/git_ui/src/commit_modal.rs

@@ -1,5 +1,7 @@
 use crate::branch_picker::{self, BranchList};
-use crate::git_panel::{GitPanel, commit_message_editor, panel_editor_style};
+use crate::git_panel::{
+    GitPanel, commit_message_editor, commit_title_exceeds_limit, panel_editor_style,
+};
 use crate::git_panel_settings::GitPanelSettings;
 use git::repository::CommitOptions;
 use git::{Amend, Commit, GenerateCommitMessage, Signoff};
@@ -548,7 +550,7 @@ impl Render for CommitModal {
                 .text(cx)
                 .lines()
                 .next()
-                .is_some_and(|title| title.len() > max_title_length)
+                .is_some_and(|title| commit_title_exceeds_limit(title, max_title_length))
         } else {
             false
         };