Merge branch 'ta/fast-import-parse-path-fix'

The way "git fast-import" handles paths described in its input has
been tightened up and more clearly documented.

* ta/fast-import-parse-path-fix:
  fast-import: make comments more precise
  fast-import: forbid escaped NUL in paths
  fast-import: document C-style escapes for paths
  fast-import: improve documentation for path quoting
  fast-import: remove dead strbuf
  fast-import: allow unquoted empty path for root
  fast-import: directly use strbufs for paths
  fast-import: tighten path unquoting