FETCH_HEAD format is undocumented

Hi,

While trying to use `git fmt-merge-msg`, I figured it only accepts
inputs in the format used in the 'FETCH_HEAD' file.

I then tried to find a description of the format of this file in the documentation, 
but could not find any. For example, 'git grep not-for-merge' in /Documentation
returns a single hit, in the release notes for Git 2.5.0.

I think it could be good if we documented the FETCH_HEAD format.

Cheers,

Philippe.