Merge branch 'jk/fast-import-unsafe'

The `--export-marks` option of `git fast-import` is exposed also via the in-stream command `feature export-marks=...` and it allows overwriting arbitrary paths. This topic branch prevents the in-stream version, to prevent arbitrary file accesses by `git fast-import` streams coming from untrusted sources (e.g. in remote helpers that are based on `git fast-import`). This fixes CVE-2019-1348. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>