Merge ~nacc/git-ubuntu:lp1708203-lint-do-not-clobber-working-tree into git-ubuntu:master

[Nish Aravamudan]

Make jenkins happy.

[Server Team CI bot (server-team-bot)]

PASSED: Continuous integration, rev:2df8748cf2a58aa40b37bc765698184abb7ae2ac
https://jenkins.ubuntu.com/server/job/git-ubuntu-ci/85/
Executed test runs:
    SUCCESS: Checkout
    SUCCESS: Style Check
    SUCCESS: Unit Tests
    SUCCESS: Integration Tests
    IN_PROGRESS: Declarative: Post Actions

Click here to trigger a rebuild:
https://jenkins.ubuntu.com/server/job/git-ubuntu-ci/85/rebuild

review: Approve (continuous-integration)

[Server Team CI bot (server-team-bot)]

PASSED: Continuous integration, rev:6455fd2a387064fafca47b7c8494a619acc3020a
https://jenkins.ubuntu.com/server/job/git-ubuntu-ci/98/
Executed test runs:
    SUCCESS: Checkout
    SUCCESS: Style Check
    SUCCESS: Unit Tests
    SUCCESS: Integration Tests
    IN_PROGRESS: Declarative: Post Actions

Click here to trigger a rebuild:
https://jenkins.ubuntu.com/server/job/git-ubuntu-ci/98/rebuild

review: Approve (continuous-integration)

[Robie Basak (racb)]

Looks good! Really tidy.

tree_hash_after_command may want to take multiple commands eventually but I suppose it's probably YAGNI right now.

review: Approve

diff --git a/gitubuntu/git_repository.py b/gitubuntu/git_repository.py
index 7ed7bdb..2c3e410 100644
--- a/gitubuntu/git_repository.py
+++ b/gitubuntu/git_repository.py
@@ -1509,3 +1509,29 @@ class GitUbuntuRepository:
         else:
             # No empty directories were added
             return tree_hash_str         # no replacement was needed
+
+    @contextmanager
+    def temporary_worktree(self, commitish):
+        with tempfile.TemporaryDirectory() as tempdir:
+            self.git_run(['worktree', 'add', '--force', tempdir, commitish])
+
+            oldcwd = os.getcwd()
+            os.chdir(tempdir)
+
+            yield
+
+            os.chdir(oldcwd)
+            self.git_run(['worktree', 'prune'])
+
+    def tree_hash_after_command(self, commitish, cmd):
+        with self.temporary_worktree(commitish):
+            try:
+                run(cmd)
+            except CalledProcessError as e:
+                logging.error("Unable to execute `%s`", ' '.join(cmd))
+                raise
+
+            run(["git", "add", "-f", ".",])
+            cp = run(["git", "write-tree"])
+            tree_hash = decode_binary(cp.stdout).strip()
+        return tree_hash
diff --git a/gitubuntu/lint.py b/gitubuntu/lint.py
index a5850a2..baa1492 100644
--- a/gitubuntu/lint.py
+++ b/gitubuntu/lint.py
@@ -440,20 +440,15 @@ def hunk_in_patch(hunk, patch):
     return False
 
 def tree_after_merge_changelogs(repo, old, new):
-    if repo.raw_repo.head_is_detached:
-        head_string = str(repo.raw_repo.head.peel(pygit2.Commit).id)
-    else:
-        head_string = repo.raw_repo.head.name[len('refs/heads/'):]
-
-    merge_base_id = str(
-        repo.raw_repo.merge_base(
-            repo.get_commitish(old).id,
-            repo.get_commitish(new).id
-        )
-    )
-    repo.checkout_commitish(new)
     changelog_files = []
     try:
+        merge_base_id = str(
+            repo.raw_repo.merge_base(
+               repo.get_commitish(old).id,
+                repo.get_commitish(new).id
+            )
+        )
+
         for tree in [merge_base_id, old, new]:
             changelog_files.append(
                 repo.extract_file_from_treeish(
@@ -461,50 +456,20 @@ def tree_after_merge_changelogs(repo, old, new):
                     "debian/changelog",
                 )
             )
-        run(["dpkg-mergechangelogs"] + [f.name for f in changelog_files] +
-            ["debian/changelog"]
-        )
-        repo.git_run(
-            ["--work-tree", ".", "add", "-f", "debian/changelog"],
-        )
-        cp = repo.git_run(
-            ["--work-tree", ".", "write-tree"],
-        )
-        return decode_binary(cp.stdout).strip()
-    except (CalledProcessError, FileNotFoundError):
-        logging.error("Unable to find/execute dpkg-mergechangelogs")
-        return None
+
+        cmd = ["dpkg-mergechangelogs",]
+        cmd.extend(f.name for f in changelog_files)
+        cmd.extend(["debian/changelog",])
+        return repo.tree_hash_after_command(new, cmd)
     finally:
         for f in changelog_files:
             f.close()
-        repo.git_run(["clean", "-f", "-d"])
-        repo.reset_commitish(new)
-        repo.checkout_commitish(head_string)
 
 def _tree_after_update_maintainer(repo, commitish_string):
-    if repo.raw_repo.head_is_detached:
-        head_string = str(repo.raw_repo.head.peel(pygit2.Commit).id)
-    else:
-        head_string = repo.raw_repo.head.name[len('refs/heads/'):]
-
-    repo.checkout_commitish(commitish_string)
-    try:
-        run(["update-maintainer"])
-    except (CalledProcessError, FileNotFoundError):
-        logging.error("Unable to find/execute update-maintainer")
-        return None
-    else:
-        repo.git_run(
-            ["--work-tree", ".", "add", "-f", "-A"],
-        )
-        cp = repo.git_run(
-            ["--work-tree", ".", "write-tree"],
-        )
-        return decode_binary(cp.stdout).strip()
-    finally:
-        repo.git_run(["clean", "-f", "-d"])
-        repo.reset_commitish(commitish_string)
-        repo.checkout_commitish(head_string)
+    return repo.tree_hash_after_command(
+        commitish_string,
+        ["update-maintainer",]
+    )
 
 def _check_update_maintainer(repo, commitish_string):
     commitish = repo.get_commitish(commitish_string)