Don't try to save every changed buffer when compiling a file

The fix for issue #193 was an improvement in that it respects `compilation-ask-about-save`, however it's surprising (and, for some, annoying) that it'll try to save every other changed buffer.

Even if compilation-ask-about-save is nil, it might be surprising to save all buffers in the context of slime-compile-file since, unlike `M-x compile`, it's only dealing with a single file.

This proposed changed still honors `compilation-ask-about-save` (and `compilation-save-buffers-predicate`) but doesn't save buffers other than the current one.

@ellerh does this sound good to you?