handle lower case drive letters on Windows

handle lower case drive letters on Windows

On Windows, if a tool calls SetCurrentDirectory with a lower case drive
letter, the subsequent call to GetCurrentDirectory will return the same
lower case drive letter. Powershell, for example, does not normalize the
path. If that happens, test-drop-caches will error out as it does not
correctly to handle lower case drive letters.

Helped-by: Johannes Schindelin <redacted>
Signed-off-by: Ben Peart <redacted>
Signed-off-by: Junio C Hamano <redacted>