fix(init): guard read_file_lines against directory paths (#189)

Problem: when a commit contains a submodule or a symlink to a directory,
`read_file_lines` passes the path to `io.open` which succeeds, then
`f:lines()` raises `"Is a directory"` inside the decoration provider.

Solution: check `vim.fn.isdirectory(path)` before `io.open` and return
nil early, which `compute_hunk_context` already handles gracefully.

README.md

@@ -61,7 +61,8 @@ luarocks install diffs.nvim
 Do not lazy load `diffs.nvim` with `event`, `lazy`, `ft`, `config`, or `keys` to
 control loading - `diffs.nvim` lazy-loads itself.
 
-**Q: Does diffs.nvim support vim-fugitive/Neogit/neojj/gitsigns?**
+**Q: Does diffs.nvim support
+[vim-fugitive](https://github.com/tpope/vim-fugitive)/[Neogit](https://github.com/NeogitOrg/neogit)/[neojj](https://github.com/NicholasZolton/neojj)/[gitsigns](https://github.com/lewis6991/gitsigns.nvim)?**
 
 Yes. Enable integrations in your config:
 

lua/diffs/init.lua

@@ -347,6 +347,9 @@ end
 ---@param path string
 ---@return string[]?
 local function read_file_lines(path)
+  if vim.fn.isdirectory(path) == 1 then
+    return nil
+  end
   local f = io.open(path, 'r')
   if not f then
     return nil

spec/context_spec.lua

@@ -165,6 +165,39 @@ describe('context', function()
       assert.is_nil(hunks[1].context_after)
     end)
 
+    it('skips when path is a directory', function()
+      vim.fn.mkdir(vim.fs.joinpath(tmpdir, 'subdir'), 'p')
+
+      local hunks = {
+        make_hunk('subdir', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+x' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('skips when path is a symlink to a directory', function()
+      vim.fn.mkdir(vim.fs.joinpath(tmpdir, 'real_dir'), 'p')
+      vim.uv.fs_symlink(vim.fs.joinpath(tmpdir, 'real_dir'), vim.fs.joinpath(tmpdir, 'link_dir'))
+
+      local hunks = {
+        make_hunk('link_dir', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+x' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
     it('skips when file does not exist on disk', function()
       local hunks = {
         make_hunk('nonexistent.lua', {