Merge branch 'ps/reflog-migrate-fixes'

"git refs migrate" to migrate the reflog entries from a refs backend to another had a handful of bugs squashed. * ps/reflog-migrate-fixes: refs: fix invalid old object IDs when migrating reflogs refs: stop unsetting REF_HAVE_OLD for log-only updates refs/files: detect race when generating reflog entry for HEAD refs: fix identity for migrated reflogs ident: fix type of string length parameter builtin/reflog: implement subcommand to write new entries refs: export `ref_transaction_update_reflog()` builtin/reflog: improve grouping of subcommands Documentation/git-reflog: convert to use synopsis type