Fix (modify/delete) conflicts in GitAlg.mergeTheirs

`merge --strategy-option=theirs` can fail if a file has been modified on
`HEAD` but deleted on the other branch. Git reports these conflicts as
`CONFLICT (modify/delete)`. This PR resolves them by just removing all
unmerged files which should be the same files that were deleted on the
other branch.

Closes #1393.

Author: fthomas

modules/core/src/main/scala/org/scalasteward/core/git/GitAlg.scala

@@ -163,8 +163,19 @@ object GitAlg {
         for {
           before <- latestSha1(repo, Branch.head)
           repoDir <- workspaceAlg.repoDir(repo)
-          sign = if (config.signCommits) List("--gpg-sign") else List.empty[String]
+          sign = if (config.signCommits) List("--gpg-sign") else List.empty
           _ <- exec(Nel.of("merge", "--strategy-option=theirs") ++ (sign :+ branch.name), repoDir)
+            .handleErrorWith { throwable =>
+              // Resolve CONFLICT (modify/delete) by deleting unmerged files:
+              for {
+                unmergedFiles <- exec(Nel.of("diff", "--name-only", "--diff-filter=U"), repoDir)
+                _ <- Nel.fromList(unmergedFiles.filter(_.nonEmpty)) match {
+                  case Some(files) => files.traverse(file => exec(Nel.of("rm", file), repoDir))
+                  case None        => F.raiseError(throwable)
+                }
+                _ <- commitAll(repo, "Remove unmerged files")
+              } yield List.empty
+            }
           after <- latestSha1(repo, Branch.head)
         } yield Option.when(before =!= after)(Commit())
 

modules/core/src/test/scala/org/scalasteward/core/git/GitAlgTest.scala

@@ -125,7 +125,7 @@ class GitAlgTest extends AnyFunSuite with Matchers {
     p.unsafeRunSync() shouldBe ((true, false, false, true))
   }
 
-  test("mergeTheirs2") {
+  test("mergeTheirs: CONFLICT (modify/delete)") {
     val repo = Repo("merge", "theirs2")
     val p = for {
       repoDir <- workspaceAlg.repoDir(repo)
@@ -171,20 +171,13 @@ object GitAlgTest {
       _ <- processAlg.exec(Nel.of("git", "add", "file2"), repoDir)
       _ <- processAlg.exec(Nel.of("git", "commit", "-m", "Modify file2 on conflicts-yes"), repoDir)
       _ <- processAlg.exec(Nel.of("git", "checkout", "master"), repoDir)
-      // work on conflicts-yes (file removed on base branch)
-      _ <- processAlg.exec(Nel.of("git", "checkout", "-b", "conflicts-yes-file-removed"), repoDir)
-      _ <- fileAlg.writeFile(repoDir / "file2", "file2, line1\nfile2, line2 on conflicts-yes")
-      _ <- processAlg.exec(Nel.of("git", "add", "file2"), repoDir)
-      _ <- processAlg.exec(Nel.of("git", "commit", "-m", "Modify file2 on conflicts-yes"), repoDir)
-      _ <- processAlg.exec(Nel.of("git", "checkout", "master"), repoDir)
       // work on master
       _ <- fileAlg.writeFile(repoDir / "file2", "file2, line1\nfile2, line2 on master")
       _ <- processAlg.exec(Nel.of("git", "add", "file2"), repoDir)
       _ <- processAlg.exec(Nel.of("git", "commit", "-m", "Modify file2 on master"), repoDir)
     } yield ()
 
-  def createGitRepoWithConflictFileRemovedOnMaster[F[_]](repoDir: File)(
-      implicit
+  def createGitRepoWithConflictFileRemovedOnMaster[F[_]](repoDir: File)(implicit
       fileAlg: FileAlg[F],
       processAlg: ProcessAlg[F],
       F: Monad[F]