tests/failfilemerge.py

merge: use the new action based mapping introduced in mergeresult obj

Before this patch, we have good number of instances of following types:

```
for f, (m, args, msg) in mresult.actions:
    if m == ACTION_X:
        do_something
```

We iterate over the whole list and then filter for a certain action. Previous
patch introduced a action based mapping in mergeresult object. The above code
now looks like:

```
for f, args, msg in mresult.getactions([ACTION_X, ...]):
    do_something
```

Differential Revision: https://phab.mercurial-scm.org/D8831

# extension to emulate interrupting filemerge._filemerge

from __future__ import absolute_import

from mercurial import (
    error,
    extensions,
    filemerge,
)


def failfilemerge(
    filemergefn, premerge, repo, wctx, mynode, orig, fcd, fco, fca, labels=None
):
    raise error.Abort("^C")
    return filemergefn(premerge, repo, mynode, orig, fcd, fco, fca, labels)


def extsetup(ui):
    extensions.wrapfunction(filemerge, '_filemerge', failfilemerge)