git: Fix branch picker creating new branches with refs/head/ prefixed on the branch name (#44206)

The bug was introduced in this recent PR:
https://github.com/zed-industries/zed/pull/42819. Since it's still in
nightly, there is no need for release notes.

I also polished the feature a bit by:
- Ensuring branch names are always a single line so the branch picker's
uniform list uses the correct element height.
- Adding tooltip text for the filter remotes button.
- Fixing the create branch from default icon showing up for non-new
branch entries.

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -1332,7 +1332,8 @@
     "context": "GitBranchSelector || (GitBranchSelector > Picker > Editor)",
     "use_key_equivalents": true,
     "bindings": {
-      "ctrl-shift-backspace": "branch_picker::DeleteBranch"
+      "ctrl-shift-backspace": "branch_picker::DeleteBranch",
+      "ctrl-shift-i": "branch_picker::FilterRemotes"
     }
   }
 ]