Stop `-t msvc -o` from lowercasing paths from /showIncludes output.

/showIncludes prints absolute paths. If a source file says
`#include <WiNdOwS.h>`, /showIncludes will use that spelling in
its output for the basename and use the on-disk cache for the
directory names in the rest for its output.

This makes the .d files created by `-t msvc -o` consistent with the
.d files written by gcc and clang.
Before this change, `-t msvc -o` would convert this output to
lower case. This is a problem if a build step produces a header file
with mixed case, such as "RuntimeFeatures.h". Due to the lowercasing,
the .d file would contain "runtimefeatures.h", while the build step
will create "RuntimeFeatures.h". Due to the case difference, ninja
would not realize that regeneration of the .h file would require
a rebuild of all source files having the header in the .d file.
(On the next build, ninja would rebuild them since stat()ing is not
case-sensitive on Windows.) One possible fix for this is to make sure
that generators always write generated header files in lower case too,
but on Mac gcc doesn't do lower-casing and .d files end up with the
case-as-written, so generators would have to be different on Mac and
Windows, which is undesirable.

If case-insensitve path comparisons are useful, they should be done
somewhere else (e.g. in CanonicalizePath()) where they affect both
paths read from .d files and paths read from .ninja files. This should
then be controlled by a top-level variable.

This patch changes behavior, but it only has an effect on generated
header files, which aren't common, and it only affects -t msvc, which
is still marked as experimental. (cmake doesn't use it yet.)

(If a file has both `#include <windows.h>` and `<Windows.h>`, this
will now take 2 stat() calls instead of just one, but that should
have a negligible cost.)
Merge pull request #589 from nico/wincase

Stop `-t msvc -o` from lowercasing paths from /showIncludes output.