git-p4: allow unshelving of branched files

git-p4: allow unshelving of branched files

When unshelving a changelist, git-p4 tries to work out the appropriate
parent commit in a given branch (default: HEAD).  To do this, it looks
at the state of any pre-existing files in the target Perforce branch,
omitting files added in the shelved changelist.  Currently, only files
added (or move targets) are classed as new.  However, files integrated
from other branches (i.e. a 'branch' action) also need to be considered
as added, for this purpose.

Signed-off-by: Simon Williams <simon@no-dns-yet.org.uk>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>