receive-pack: detect aliased updates which can occur with symrefs

receive-pack: detect aliased updates which can occur with symrefs

When pushing to a remote repo the sending side filters out aliased
updates (e.g., foo:baz bar:baz). However, it is not possible for the
sender to know if two refs are aliased on the receiving side via
symrefs. Here is one such scenario:

  $ git init origin
  $ (cd origin && touch file && git add file && git commit -a -m intial)
  $ git clone --bare origin origin.git
  $ rm -rf origin

  $ git clone origin.git client

  $ git clone --mirror client backup.git &&
  $ (cd backup.git && git remote set-head origin --auto)

  $ (cd client &&
	git remote add --mirror backup ../backup.git &&
	echo change1 > file && git commit -a -m change1 &&
	git push origin &&
	git push backup
	)

The push to backup fails with:

  Counting objects: 5, done.
  Writing objects: 100% (3/3), 244 bytes, done.
  Total 3 (delta 0), reused 0 (delta 0)
  Unpacking objects: 100% (3/3), done.
  error: Ref refs/remotes/origin/master is at ef3... but expected 262...
  remote: error: failed to lock refs/remotes/origin/master
  To ../backup.git
     262cd57..ef307ff  master -> master
     262cd57..ef307ff  origin/HEAD -> origin/HEAD
   ! [remote rejected] origin/master -> origin/master (failed to lock)
  error: failed to push some refs to '../backup.git'

The reason is that refs/remotes/origin/HEAD is a symref to
refs/remotes/origin/master, but it is not possible for the sending side
to unambiguously know this.

This commit fixes the issue by having receive-pack ignore any update to
a symref whose target is being identically updated. If a symref and its
target are being updated inconsistently, then the update for both fails
with an error message ("refusing inconsistent update...") to help
diagnose the situation.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>