sync: push current branch first

sync: push current branch first

sync: Push the current branch first, rather than a synced branch, to better
support git forges (gitlab, gitea, forgejo, etc.) which use push-to-create
with the first pushed branch becoming the default branch.

With considerable complication to filter out warning message about
receive.denyCurrentBranch when pushing to a non-bare repository. Localization
may break it in the future, but it seems like the best way to handle this. See
my comments for the gory details.