builtin-remote: make rm operation safer in mirrored repository

builtin-remote: make rm operation safer in mirrored repository

"git remote rm <repo>" happily removes non-remote refs and their reflogs.
This may be okay if the repository truely is a mirror, but if the user
had done "git remote add --mirror <repo>" by accident and was just
undoing their mistake, then they are left in a situation that is
difficult to recover from.

After this commit, "git remote rm" skips over non-remote refs. The user
is advised on how remove branches using "git branch -d", which itself
has nice safety checks wrt to branch removal lacking from "git remote rm".
Non-remote non-branch refs are skipped silently.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>