config: New command for storing configuration in the git-annex branch.

config: New command for storing configuration in the git-annex branch.

Any config names can be set using this; git-annex commands will only look
at specific ones that make sense and are worth the overhead of querying the
branch.

This might also be useful for storing whatever other config-type stuff the
user might want to shove into the git-annex branch.

This commit was sponsored by Jochen Bartl on Patreon.