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"