I have a script running some Mercurial commands in non-interactive mode on a build server. One of the commands merges two branches and there is always a conflict in the `.hgtags` file during the merge because of the way the build scripts are set up.

How can I force Mercurial to always merge the `.hgtags` file using changes from both files, first from one, then from the other?

For example, if I the files to merge were

```
A
B
C

```

and

```
A
B
D

```

I would like the result to be

```
A
B
C
D

```

I am guessing I will need a custom merge tool. What tool provides this functionality?