Merge branch 'en/fast-export-encoding'

The "git fast-export/import" pair has been taught to handle commits
with log messages in encoding other than UTF-8 better.

* en/fast-export-encoding:
  fast-export: do automatic reencoding of commit messages only if requested
  fast-export: differentiate between explicitly UTF-8 and implicitly UTF-8
  fast-export: avoid stripping encoding header if we cannot reencode
  fast-import: support 'encoding' commit header
  t9350: fix encoding test to actually test reencoding