feat: support for detached working trees

Added config.worktrees.

Array of tables with the keys 'gitdir' and 'toplevel'.

If attaching normally fails, then each entry in the table is attempted.

Example:

  worktrees = {
    {
      toplevel = vim.env.HOME,
      gitdir = vim.env.HOME .. '/projects/dotfiles/.git'
    }
  }

Resolves #397

README.md

@@ -28,6 +28,7 @@ Super fast git decorations implemented purely in lua/teal.
 - Live intra-line word diff
 - Ability to display deleted/changed lines via virtual lines.
 - Support for [yadm](https://yadm.io/)
+- Support for detached working trees.
 
 ## Requirements
 

doc/gitsigns.txt

@@ -518,6 +518,24 @@ keymaps                                              *gitsigns-config-keymaps*
       to `{}`, and |gitsigns-config-on_attach| can instead be used to define
       mappings.
 
+worktrees                                          *gitsigns-config-worktrees*
+      Type: `table`, Default: `nil`
+
+      Detached working trees.
+
+      Array of tables with the keys `gitdir` and `toplevel`.
+
+      If normal attaching fails, then each entry in the table is attempted
+      with the work tree details set.
+
+      Example: >
+        worktrees = {
+          {
+            toplevel = vim.env.HOME,
+            gitdir = vim.env.HOME .. '/projects/dotfiles/.git'
+          }
+        }
+
 on_attach                                          *gitsigns-config-on_attach*
       Type: `function`, Default: `nil`