LibGit2 miscellaneous fixes to credential callback

[omus]

Part of #20725.

• Removes unnecessary nothing checks against Strings.
• Corrects a type instability with the err variable.
• Uses @static to avoid runtime calls to is_windows

[tkelman]

where's the type instability here?

is doing is_windows at runtime really making a material difference?

[omus]

Type instability was that err = 0 is an Int64 and later it is set to a Int32 (Cint) by authenticate_ssh. Also note that err == 0 && return Cint(0) at the end of authenticate_ssh makes that function return either false or Cint(0)

[omus]

is doing is_windows at runtime really making a material difference?

Probably not, I didn't do performance tests between the two. Is there an issue with adding in the @static macro?

[tkelman]

same as other kinds of conditional compilation, you could have invalid code in the inactive branch and possibly not notice until you send things through appveyor

[omus]

I didn't consider that. Since we always test with appveyor though the invalid code will eventually get caught. I personally would still leave in the @static macro. I'd be okay with dropping that commit if that is your preference.

[tkelman]

appveyor doesn't help people testing locally, I'd rather use @static as sparingly as possible, only where absolutely necessary for performance or failure to build without it

[yuyichao]

What's the kind of error you want to catch by removing @static?

[ararslan]

I understand your point and agree in principle, @tkelman, but it seems pretty harmless in this case. (At least it seems that way to me.) I have no strong feelings either way I suppose, but the rest of this PR looks good to me.

[tkelman]

Deprecations, as one example

julia> @static if false
           ASCIIString("foo")
       end

julia> if false
           ASCIIString("foo")
       end
WARNING: Base.ASCIIString is deprecated, use String instead.
  likely near no file:0

Unless you're actually looking at the appveyor log every time, that stuff can slip through until someone who actually builds on windows (usually me) notices and fixes it. If @static isn't necessary, I'd rather have that failing your local builds first (if it's running in --depwarn=error anyway), even on other operating systems before you submit a PR and waste a bunch of CI time.

[omus]

I've reverted the @static change.