git: Raise WARNS in secure/ as high as possible and fix some resulting warnings.

[Tobias Weingartner]

On Monday, January 18, Sascha Wildner wrote:
> 
> I'm not sure why gcc warns in x86_64 about trying to printf a uint64_t 
> using %llu. To me it seems like a gcc bug/quirk. If anyone knows how to 
> change this in the compiler (or can provide more insight), it would be 
> welcome.

Because on x86_64 a uint64_t is 'unsigned long int', not
'unsigned long long'.  Yes, they may be the "same", but
for typing purposes they are not...

--Toby.