Re: FETCH_HEAD format is undocumented

Philippe Blain [off-list ref] writes:

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.

Not just the format of FETCH_HEAD file, but the whole usage of
fmt-merge-msg is meant as a mere implementation detail of the
one-way communication from "git fetch" to "git merge".  I'd rather
leave it opaque so that we can update the format as needed.  For one
thing it is impossible to do i18n with the current format, I think,
which some folks may eventually want to change.

I am OK with documenting it somewhere in Documentation/technical/
with a warning written in big red letters that it is subject to
change, to clearly mark that the format of the file should not be
relied upon and the only useful thing you could do is to rely on the
fact that its first 40-byte gives one object name (so you could say
"git log ...FETCH_HEAD" for example) and to feed it to "git merge".