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/config/highlights.lua

@@ -49,6 +49,7 @@ Snacks.util.set_hl({
   UndoSaved = "Special",
   GitCommit = "@variable.builtin",
   GitBreaking = "Error",
+  GitDetached = "DiagnosticWarn",
   GitBranch = "Title",
   GitBranchCurrent = "Number",
   GitDate = "Special",

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] = { " " }

lua/snacks/picker/preview.lua

@@ -192,7 +192,7 @@ function M.cmd(cmd, ctx, opts)
       if not killed and code ~= 0 then
         Snacks.notify.error(
           ("Terminal **cmd** `%s` failed with code `%d`:\n- `vim.o.shell = %q`\n\nOutput:\n%s"):format(
-            cmd,
+            type(cmd) == "table" and table.concat(cmd, " ") or cmd,
             code,
             vim.o.shell,
             vim.trim(table.concat(output, ""))
@@ -254,7 +254,7 @@ function M.git_log(ctx)
     "--color=never",
     "--no-show-signature",
     "--no-patch",
-    ctx.item.branch,
+    ctx.item.commit,
   }
   if not native then
     table.insert(cmd, 2, "--no-pager")

lua/snacks/picker/source/git.lua

@@ -163,18 +163,38 @@ function M.branches(opts)
   local args = { "--no-pager", "branch", "--no-color", "-vvl" }
   local cwd = vim.fs.normalize(opts and opts.cwd or uv.cwd() or ".") or nil
   cwd = Snacks.git.get_root(cwd)
+
+  local pattern_hash = "[a-zA-Z0-9]+"
+  local patterns = {
+    -- stylua: ignore start
+    --- e.g. "* (HEAD detached at f65a2c8) f65a2c8 chore(build): auto-generate docs"
+    "^(.)%s(%(HEAD detached at " .. pattern_hash .. "%))%s+(" .. pattern_hash .. ")%s*(.*)$",
+    --- e.g. "  main                       d2b2b7b [origin/main: behind 276] chore(build): auto-generate docs"
+    "^(.)%s(%S+)%s+(".. pattern_hash .. ")%s*(.*)$",
+    -- stylua: ignore end
+  } ---@type string[]
+
   return require("snacks.picker.source.proc").proc(vim.tbl_deep_extend("force", {
     cwd = cwd,
     cmd = "git",
     args = args,
     ---@param item snacks.picker.finder.Item
     transform = function(item)
-      local status, branch, commit, msg = item.text:match("^(.)%s(%S+)%s+([a-zA-Z0-9]+)%s*(.*)$")
       item.cwd = cwd
-      item.current = status == "*"
-      item.branch = branch
-      item.commit = commit
-      item.msg = msg
+      for p, pattern in ipairs(patterns) do
+        local status, branch, commit, msg = item.text:match(pattern)
+        if status then
+          local detached = p == 1
+          item.current = status == "*"
+          item.branch = not detached and branch or nil
+          item.commit = commit
+          item.msg = msg
+          item.detached = detached
+          return
+        end
+      end
+      Snacks.notify.warn("failed to parse branch: " .. item.text)
+      return false -- skip items we could not parse
     end,
   }, opts or {}))
 end