Allow deleting a merge commit

For non-merge commits we change "pick" to "drop" when we delete them. We do this
so that we can use the same code for dropping a commit no matter whether we are
in an interactive rebase or not. (If we aren't, we could just as well delete the
pick line from the todo list instead of setting it to "drop", but if we are, it
is better to keep the line around so that the user can change it back to "pick"
if they change their mind.)

However, merge commits can't be changed to "drop", so we have to delete them
from the todo file. We add a new daemon instruction that does this.

We still don't allow deleting a merge commit from within an interactive rebase.
The reason is that we don't show the "label" and "reset" todos in lazygit, so
deleting a merge commit would leave the commits from the branch that is being
merged in the list as "pick" commits, with no indication that they are going to
be dropped because they are on a different branch, and the merge commit that
would have brought them in is gone. This could be very confusing.

pkg/app/daemon/daemon.go

@@ -38,6 +38,7 @@ const (
 	DaemonKindMoveTodosDown
 	DaemonKindInsertBreak
 	DaemonKindChangeTodoActions
+	DaemonKindDropMergeCommit
 	DaemonKindMoveFixupCommitDown
 	DaemonKindWriteRebaseTodo
 )
@@ -57,6 +58,7 @@ func getInstruction() Instruction {
 		DaemonKindRemoveUpdateRefsForCopiedBranch: deserializeInstruction[*RemoveUpdateRefsForCopiedBranchInstruction],
 		DaemonKindCherryPick:                      deserializeInstruction[*CherryPickCommitsInstruction],
 		DaemonKindChangeTodoActions:               deserializeInstruction[*ChangeTodoActionsInstruction],
+		DaemonKindDropMergeCommit:                 deserializeInstruction[*DropMergeCommitInstruction],
 		DaemonKindMoveFixupCommitDown:             deserializeInstruction[*MoveFixupCommitDownInstruction],
 		DaemonKindMoveTodosUp:                     deserializeInstruction[*MoveTodosUpInstruction],
 		DaemonKindMoveTodosDown:                   deserializeInstruction[*MoveTodosDownInstruction],
@@ -242,6 +244,30 @@ func (self *ChangeTodoActionsInstruction) run(common *common.Common) error {
 	})
 }
 
+type DropMergeCommitInstruction struct {
+	Hash string
+}
+
+func NewDropMergeCommitInstruction(hash string) Instruction {
+	return &DropMergeCommitInstruction{
+		Hash: hash,
+	}
+}
+
+func (self *DropMergeCommitInstruction) Kind() DaemonKind {
+	return DaemonKindDropMergeCommit
+}
+
+func (self *DropMergeCommitInstruction) SerializedInstructions() string {
+	return serializeInstruction(self)
+}
+
+func (self *DropMergeCommitInstruction) run(common *common.Common) error {
+	return handleInteractiveRebase(common, func(path string) error {
+		return utils.DropMergeCommit(path, self.Hash, getCommentChar())
+	})
+}
+
 // Takes the hash of some commit, and the hash of a fixup commit that was created
 // at the end of the branch, then moves the fixup commit down to right after the
 // original commit, changing its type to "fixup" (only if ChangeToFixup is true)