Merge branch 'cc/fast-import-usage' into seen

Merge branch 'cc/fast-import-usage' into seen

The usage string of 'git fast-import' has been updated to use the
parse_options() API for displaying help, and its SYNOPSIS in the
documentation has been standardized to match.

* cc/fast-import-usage:
  fast-import: use struct option for usage string
  fast-import: move command state globals into 'struct fast_import_state'
  fast-import: introduce 'struct fast_import_state'
  fast-import: localize 'i' into the 'for' loops using it
  api-parse-options.adoc: document hidden and OPT_*_F option macros
  api-parse-options.adoc: document per-option flags
  parse-options: introduce OPT_HIDDEN_GROUP