support commit.gpgsign

Support users who have set commit.gpgsign, by disabling gpg signatures for
git-annex branch commits and commits made by the assistant.

The thinking here is that a user sets commit.gpgsign intending the commits
that they manually initiate to be gpg signed. But not commits made in the
background, whether by a deamon or implicitly to the git-annex branch.
gpg signing those would be at best a waste of CPU and at worst would fail,
or flood the user with gpg passphrase prompts, or put their signature on
changes they did not directly do.

See Debian bug #753720.

Also makes all commits done by git-annex go through a few central control
points, to make such changes easier in future.

Also disables commit.gpgsign in the test suite.

This commit was sponsored by Antoine Boegli.

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