bug#63261: Recent changes to git config cause errors for non-committers

[Brian Cully]

I've run into two issues with the recent changes to git config 
integration:

1) All commits must now be signed, even if you're not a 
committer. This breaks just tons of things, including 
rebasing. I'm not sure how to fix this without just disabling that 
configuration line altogether.

2) Some ‘make’ rules now require git to be installed so that ‘git 
config’ can add ‘etc/git/gitconfig’ to the local 
configuration. So, for instance, ‘guix shell --pure -D guix -- 
make’ will now fail. Calls to git should be prefixed with a test 
to see if there is a git executable in the path.

-bjc