…UnsetSection commands. These commands operate on config stored at system scope e.g. /etc/gitconfig.

…bal if running as the root user (unless --local is specified), using the new System-level config commands.

…s to Debian deb. These will run as root and so will operate at the new --system scope.

…pts. These will run as root and so will operate at the new --system scope.

… post-install scripts in rpm/deb packages.

ExitError conditions now return a plain error with message like:

    Error running git [config --system filter.lfs.clean git-lfs clean -- %f]: 'error: could not lock config file /etc/gitconfig: Permission denied' 'exit status 255'

Also:

* Made Set/Unset Config commands return errors when they occur.
* Made install attribute set fail properly with an error message e.g. if permission-denied.  Previously this failed silently and reported success.

***NOTE*** this commit contains Go 1.6-specific APIs.  The following commit will remove those, and that following commit can be reverted when Go >= 1.6 is required.

…. This commit can be reverted as soon as Go >= 1.6 is required.

This commit copies-down some utility methods introduced as part of the API change in Go 1.6.  These changes are convenient to use here, since they auto-truncate stderr error output.

Revert this commit when moving to Go 1.6.

…xec stops swallowing the error return. Related test "ls-files: with zero files".