Merge branch 'ef/fast-export'

* ef/fast-export:
  builtin-fast-export.c: handle nested tags
  builtin-fast-export.c: fix crash on tagged trees
  builtin-fast-export.c: turn error into warning
  test-suite: adding a test for fast-export with tag variants

diff --cc builtin-fast-export.c
Simple merge

diff --cc t/t9301-fast-export.sh
Simple merge