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>

modules/git/repo.go

@@ -278,10 +278,10 @@ type DivergeObject struct {
 }
 
 // GetDivergingCommits returns the number of commits a targetBranch is ahead or behind a baseBranch
-func GetDivergingCommits(ctx context.Context, repoPath, baseBranch, targetBranch string) (do DivergeObject, err error) {
+func GetDivergingCommits(ctx context.Context, repoPath, baseBranch, targetBranch string, env []string) (do DivergeObject, err error) {
 	cmd := NewCommand(ctx, "rev-list", "--count", "--left-right").
 		AddDynamicArguments(baseBranch + "..." + targetBranch).AddArguments("--")
-	stdout, _, err := cmd.RunStdString(&RunOpts{Dir: repoPath})
+	stdout, _, err := cmd.RunStdString(&RunOpts{Dir: repoPath, Env: env})
 	if err != nil {
 		return do, err
 	}