test(git): add regression tests with a minimal headless harness

test/git/util_test.lua

@@ -0,0 +1,47 @@
+local h = require("helpers")
+local util = require("git.util")
+
+h.test("set_buf_lines preserves modifiable=false", function()
+    local buf = vim.api.nvim_create_buf(false, true)
+    vim.bo[buf].modifiable = false
+    util.set_buf_lines(buf, 0, -1, { "a", "b", "c" })
+    h.eq(
+        vim.api.nvim_buf_get_lines(buf, 0, -1, false),
+        { "a", "b", "c" },
+        "lines should be replaced"
+    )
+    h.falsy(vim.bo[buf].modifiable, "modifiable should stay false")
+    h.falsy(vim.bo[buf].modified, "modified should be cleared")
+    vim.api.nvim_buf_delete(buf, { force = true })
+end)
+
+h.test("set_buf_lines preserves modifiable=true", function()
+    local buf = vim.api.nvim_create_buf(false, true)
+    vim.bo[buf].modifiable = true
+    util.set_buf_lines(buf, 0, -1, { "a", "b" })
+    h.truthy(vim.bo[buf].modifiable, "modifiable should stay true")
+    h.falsy(vim.bo[buf].modified, "modified should be cleared")
+    vim.api.nvim_buf_delete(buf, { force = true })
+end)
+
+h.test("set_buf_lines partial range update", function()
+    local buf = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(buf, 0, -1, false, { "a", "b", "c", "d" })
+    util.set_buf_lines(buf, 1, 3, { "X", "Y", "Z" })
+    h.eq(
+        vim.api.nvim_buf_get_lines(buf, 0, -1, false),
+        { "a", "X", "Y", "Z", "d" },
+        "lines [1, 3) should be replaced"
+    )
+    vim.api.nvim_buf_delete(buf, { force = true })
+end)
+
+h.test("set_buf_lines errors on out-of-bounds (strict_indexing)", function()
+    local buf = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(buf, 0, -1, false, { "a", "b" })
+    local ok = pcall(util.set_buf_lines, buf, 100, 200, { "x" })
+    h.falsy(ok, "out-of-bounds index should error")
+    vim.api.nvim_buf_delete(buf, { force = true })
+end)
+
+h.report()