git-p4: place temporary refs used for branch import under refs/git-p4-tmp

git-p4: place temporary refs used for branch import under refs/git-p4-tmp

Git-P4 used to place temporary refs under "git-p4-tmp". Since 3da1f37
Git checks that all refs are placed under "refs". Instruct Git-P4 to
place temporary refs under "refs/git-p4-tmp". There are no backwards
compatibility considerations as these refs are transient.

Use "git show-ref --verify" to check the (non-)existience of the refs
instead of file checks assuming the file-based ref backend.

All refs under "refs" are shared across all worktrees. This is not
desired for temporary Git-P4 refs and will be adressed in a later patch.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Reviewed-by: Vitor Antunes <vitor.hda@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>