Merge branch 'ab/c99-designated-initializers'

Merge branch 'ab/c99-designated-initializers'

Use designated initializers we started using in mid 2017 in more
parts of the codebase that are relatively quiescent.

* ab/c99-designated-initializers:
  fast-import.c: use designated initializers for "partial" struct assignments
  refspec.c: use designated initializers for "struct refspec_item"
  convert.c: use designated initializers for "struct stream_filter*"
  userdiff.c: use designated initializers for "struct userdiff_driver"
  archive-*.c: use designated initializers for "struct archiver"
  object-file: use designated initializers for "struct git_hash_algo"
  trace2: use designated initializers for "struct tr2_dst"
  trace2: use designated initializers for "struct tr2_tgt"
  imap-send.c: use designated initializers for "struct imap_server_conf"