feat: improved performances when checking for conflicts on pull requests (#7727)

- `testPatch` is a function that is called to test a pull request and determine the state of the pull request. Checking for merge conflicts, check if the diff is empty and if the pull request modifies any protected files.
- The checking for merge conflict and if the diff is empty used git commands that relied on a working tree to correctly functions. Forgejo store repositories in a bare format which do not contain a working tree. This means that a temporary copy was created every time a pull request had to be re-checked and for large repositories involving quite some I/O interaction.
- This patch adjusts those codepaths to instead use newer Git plumbing commands that work without requiring a work tree and can thus be used directly on the bare repository. The merge conflict is now done via [`git-merge-tree(1)`](https://git-scm.com/docs/git-merge-tree/) and checking if the diff is empty is done via [`git-diff-tree(1)`](https://git-scm.com/docs/git-diff-tree).
- If the function is called to test a patch where the head and base repository are not the same, then [Git alternate](https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefalternateobjectdatabaseaalternateobjectdatabase) is used to make the head commit available in the base repository, this done on a per git command basis via the `GIT_ALTERNATE_OBJECT_DIRECTORIES` environment.
- As far as I can understand the documentation and the existing code, there's no edge case that the new code cannot handle. It also results in a cleaner codepath, as the existing code did a lot of checking and merging in a more traditional approach that required a lot of (parsing) code, while the new code offloads this to git and has a trivial parser of the output.
- Resolves forgejo/forgejo#7701
- Added exhaustive integration testing.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7727
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

services/pull/patch_test.go

@@ -0,0 +1,62 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package pull
+
+import (
+	"fmt"
+	"testing"
+
+	issues_model "forgejo.org/models/issues"
+	"forgejo.org/models/unittest"
+	"forgejo.org/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadHeadRevision(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+
+	t.Run("AGit", func(t *testing.T) {
+		t.Run("New", func(t *testing.T) {
+			ctx := &testPatchContext{}
+			require.NoError(t, ctx.LoadHeadRevision(t.Context(), &issues_model.PullRequest{Flow: issues_model.PullRequestFlowAGit, HeadCommitID: "Commit!"}))
+
+			assert.Empty(t, ctx.env)
+			assert.Equal(t, "Commit!", ctx.headRev)
+			assert.True(t, ctx.headIsCommitID)
+		})
+		t.Run("Existing", func(t *testing.T) {
+			ctx := &testPatchContext{}
+			require.NoError(t, ctx.LoadHeadRevision(t.Context(), &issues_model.PullRequest{Flow: issues_model.PullRequestFlowAGit, Index: 371}))
+
+			assert.Empty(t, ctx.env)
+			assert.Equal(t, "refs/pull/371/head", ctx.headRev)
+			assert.False(t, ctx.headIsCommitID)
+		})
+	})
+
+	t.Run("Same repository", func(t *testing.T) {
+		ctx := &testPatchContext{}
+		require.NoError(t, ctx.LoadHeadRevision(t.Context(), unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 1})))
+
+		assert.Empty(t, ctx.env)
+		assert.Equal(t, "refs/heads/branch1", ctx.headRev)
+		assert.False(t, ctx.headIsCommitID)
+	})
+
+	t.Run("Across repository", func(t *testing.T) {
+		pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 3})
+		require.NoError(t, pr.LoadHeadRepo(t.Context()))
+
+		ctx := &testPatchContext{}
+		require.NoError(t, ctx.LoadHeadRevision(t.Context(), pr))
+
+		if assert.NotEmpty(t, ctx.env) {
+			assert.Equal(t, fmt.Sprintf("GIT_ALTERNATE_OBJECT_DIRECTORIES=%s/user13/repo11.git/objects", setting.RepoRootPath), ctx.env[len(ctx.env)-1])
+		}
+		assert.Equal(t, "0abcb056019adb8336cf9db3ad9d9cf80cd4b141", ctx.headRev)
+		assert.True(t, ctx.headIsCommitID)
+	})
+}