Ignore executable bit when adding files if filemode=0.

Ignore executable bit when adding files if filemode=0.

If the user has configured core.filemode=0 then we shouldn't set
the execute bit in the index when adding a new file as the user
has indicated that the local filesystem can't be trusted.

This means that when adding files that should be marked executable
in a repository with core.filemode=0 the user must perform a
'git update-index --chmod=+x' on the file before committing the
addition.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>