Move implementation of <file_change_detection.h> into a C file

Move implementation of <file_change_detection.h> into a C file

file_change_detection_for_stat partially initialize
struct file_change_detection in some cases, when the size member
alone determines the outcome of all comparisons.  This results
in maybe-uninitialized compiler warnings in case of sufficiently
aggressive inlining.

Once the implementation is moved into a separate C file, this kind
of inlining is no longer possible, so the compiler warnings are gone.