fix(picker.git_branches): handle detached HEAD (#671)

## Description

Handle [detached
HEAD](https://git-scm.com/docs/git-checkout#_detached_head)

- list: display correct information
- preview: without error notification
- action: checkout the commit that HEAD detached at

<!-- Describe the big picture of your changes to communicate to the
maintainers
  why we should accept this pull request. -->

## Related Issue(s)

<!--
  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
-->

- Fixes #672 

## Screenshots

<!-- Add screenshots of the changes if applicable. -->

Before:
<img width="1308" alt="image"
src="https://github.com/user-attachments/assets/fad291a3-a730-4a2b-9eb2-4a0edd83d794"
/>

After:
<img width="1038" alt="image"
src="https://github.com/user-attachments/assets/d312579a-e12d-4286-845c-a706d91a6c95"
/>

---------

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/snacks/picker/format.lua

@@ -138,9 +138,12 @@ function M.git_branch(item, picker)
   local ret = {} ---@type snacks.picker.Highlight[]
   if item.current then
     ret[#ret + 1] = { a("", 2), "SnacksPickerGitBranchCurrent" }
-    ret[#ret + 1] = { a(item.branch, 30, { truncate = true }), "SnacksPickerGitBranch" }
   else
     ret[#ret + 1] = { a("", 2) }
+  end
+  if item.detached then
+    ret[#ret + 1] = { a("(detached HEAD)", 30, { truncate = true }), "SnacksPickerGitDetached" }
+  else
     ret[#ret + 1] = { a(item.branch, 30, { truncate = true }), "SnacksPickerGitBranch" }
   end
   ret[#ret + 1] = { " " }