tests/blacklists/linux-vfat

test: add a push race case where each client replaces a different head

We check case where the raced push replace one head while the racing push
replaces another unrelated one.

That second test also make sure we synchronise all repositories to the same
state between tests. That will help us when allowing some sort of concurrent
pushes.

# invalid filenames
test-add.t
test-init.t
test-clone.t
test-contrib.t
test-hgweb-raw.t
test-walk.t

# no sockets or fifos
test-hup.t

# no hardlinks
test-hardlinks.t
test-relink.t

# exec bit problems
test-convert-bzr-114.t
test-convert-bzr-directories.t
test-convert-bzr-merges.t
test-convert-bzr-treeroot.t
test-convert-darcs.t
test-merge-tools.t

# debugstate exec bit false positives
test-dirstate.t
test-filebranch.t
test-merge-remove.t