Retain given color diagnostics options when forcing colored output

Retain given color diagnostics options when forcing colored output

ccache currently filters out both -fdiagnostics-color and
-fcolor-diagnostics options when adding -fdiagnostics-color (GCC) or
-fcolor-diagnostics (Clang) to force colored output. The idea in #224
was that only -fdiagnostics-color options should be filtered out when
the compiler is GCC, but -fcolor-diagnostics is also removed, something
which was missed in the code review. This has the unintended side effect
that “ccache gcc -fcolor-diagnostics -c example.c” works since ccache in
effect removes -fcolor-diagnostics in the actual call to the compiler.

The bug can fixed by removing only the compiler-specific options when
forcing colored output, but a more robust method would be to retain all
color diagnostics options as is but exclude them from the input hash.
This commit makes that change and also simplifies the logic for color
diagnostics option handling.

Fixes #711.