Replicate `lfs/gitscanner_cmd` in `subprocess/buffered_cmd` using
subprocess/cmd. In subsequent commits we will migrate all functions
users of `lfs/gitscanner_cmd` to `subprocess/buffered_cmd` and
afterwards remove `lfs/gitscanner_cmd`.
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.
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.
