builtin/remote.c: show progress when renaming remote references

When renaming a remote, Git needs to rename all remote tracking
references to the remote's new name (e.g., renaming
"refs/remotes/old/foo" to "refs/remotes/new/foo" when renaming a remote
from "old" to "new").

This can be somewhat slow when there are many references to rename,
since each rename is done in a separate call to rename_ref() as opposed
to grouping all renames together into the same transaction. It would be
nice to execute all renames as a single transaction, but there is a
snag: the reference transaction backend doesn't support renames during a
transaction (only individually, via rename_ref()).

The reasons there are described in more detail in [1], but the main
problem is that in order to preserve the existing reflog, it must be
moved while holding both locks (i.e., on "oldname" and "newname"), and
the ref transaction code doesn't support inserting arbitrary actions
into the middle of a transaction like that.

As an aside, adding support for this to the ref transaction code is
less straightforward than inserting both a ref_update() and ref_delete()
call into the same transaction. rename_ref()'s special handling to
detect D/F conflicts would need to be rewritten for the transaction code
if we wanted to proactively catch D/F conflicts when renaming a
reference during a transaction. The reftable backend could support this
much more readily because of its lack of D/F conflicts.

Instead of a more complex modification to the ref transaction code,
display a progress meter when running verbosely in order to convince the
user that Git is doing work while renaming a remote.

This is mostly done as-expected, with the minor caveat that we
intentionally count symrefs renames twice, since renaming a symref takes
place over two separate calls (one to delete the old one, and another to
create the new one).

[1]: https://lore.kernel.org/git/572367B4.4050207@alum.mit.edu/

Suggested-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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