Probe filesystem and set core.filemode correctly

Probe filesystem and set core.filemode correctly

When creating a new FileRepository, probe the capability of the
local filesystem and set core.filemode based on how it reacts.

We can't just rely on FS.supportsExecute() because a POSIX system
(which usually does support execute) might be storing the repository
on a partition that doesn't have execute support (e.g. plain FAT-32).

Creating a temporary file, setting both states, checking we get
the desired results will let us set the variable correctly on
all systems.

Change-Id: I551488ea8d352d2179c7b244f474d2e3d02567a2
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>