cygwin: trust executable bit by default

cygwin: trust executable bit by default

Earlier 7974843 (compat/cygwin.c: make runtime detection of lstat/stat
lessor impact, 2008-10-23) fixed the low-level "do we use cygwin specific
hacks for stat/lstat?" logic not to call into git_default_config() from
random codepaths that are typically very late in the program, to prevent
the call from potentially overwriting other variables that are initialized
from the configuration.

However, it forgot that on Cygwin, trust-executable-bit should default to
true.

Noticed by J6t, confirmed by Ramsay Jones, and the brown paper bag is on
Gitster's head.

Signed-off-by: Junio C Hamano <gitster@pobox.com>