How can I config `git` to use `p4merge` as my `mergetool`?

Next is my current situation:

```
$ git --version
git version 1.8.3.msysgit.0

$ git mergetool

This message is displayed because 'merge.tool' is not configured.
See 'git mergetool --tool-help' or 'git help config' for more details.
'git mergetool' will now attempt to use one of the following tools:
tortoisemerge emerge vimdiff
No known merge tool is available.

```