git-p4: Detect changes to executable bit and include them in p4 submit.

git-p4: Detect changes to executable bit and include them in p4 submit.

This changeset takes advantage of the new parseDiffTreeEntry(...) function to
detect changes to the execute bit in the git repository.  During submit, git-p4
now looks for changes to the executable bit and if it finds them it "reopens"
the file in perforce, which allows it to change the file type.

The logic for adding the executable bit in perforce is straightforward: the +x
modifier can be used. Removing the executable bit in perforce requires that the
entire filetype be redefined (there is no way to join remove the bit with a -x
modifier, for example). This changeset includes logic to remove the executable
bit from the full file type while preserving the base file type and other
modifiers.

Signed-off-by: Chris Pettitt <cpettitt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>