fix(git): show merge commits diffed against the first parent

lua/git/object.lua

@@ -184,8 +184,7 @@ local function populate(buf, r, rev, state, rev_sha)
             local patch = util.git({
                 "diff-tree",
                 "-p",
-                "-m",
-                "--first-parent",
+                "--diff-merges=first-parent",
                 "--root",
                 "--no-commit-id",
                 commit_sha,

test/git/object_test.lua

@@ -86,6 +86,23 @@ t.test("M.open(HEAD:<path>) loads file content at HEAD", function()
     )
 end)
 
+t.test("M.open on a merge commit diffs against the first parent only", function()
+    local dir = h.make_repo({ ["a.txt"] = "one\n" })
+    t.write(dir, "a.txt", "two\n")
+    h.git(dir, "stash")
+    local stash = h.git(dir, "rev-parse", "stash@{0}").stdout
+    local r = assert(require("git.core.repo").resolve(dir))
+
+    object.open(r, stash, { split = false })
+    local count = 0
+    for _, l in ipairs(vim.api.nvim_buf_get_lines(0, 0, -1, false)) do
+        if l:match("^diff %-%-git ") then
+            count = count + 1
+        end
+    end
+    t.eq(count, 1, "the stashed file's diff appears once, not per-parent")
+end)
+
 t.test("M.open errors on a bogus base, no buffer is opened", function()
     local dir = h.make_repo({ a = "first\n" })
     local r = assert(require("git.core.repo").resolve(dir))