[PATCH v3] 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 <johannes.schindelin@gmx.de>
Signed-off-by: Ben Peart <Ben.Peart@microsoft.com>
---

Notes:
    Base Ref: master
    Web-Diff: https://github.com/benpeart/git/commit/1ff8de1b6c
    Checkout: git fetch https://github.com/benpeart/git drop-caches-v3 && git checkout 1ff8de1b6c
    
    ### Interdiff (v2..v3):
    
    diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
    index 37047189c3..f65e301f9d 100644
    --- a/t/helper/test-drop-caches.c
    +++ b/t/helper/test-drop-caches.c
    @@ -16,8 +16,8 @@ static int cmd_sync(void)
     	if ((0 == dwRet) || (dwRet > MAX_PATH))
     		return error("Error getting current directory");
    
    -	if ((toupper(Buffer[0]) < 'A') || (toupper(Buffer[0]) > 'Z'))
    -		return error("Invalid drive letter '%c'", Buffer[0]);
    +	if (!has_dos_drive_prefix(Buffer))
    +		return error("'%s': invalid drive letter", Buffer);
    
     	szVolumeAccessPath[4] = Buffer[0];
     	hVolWrite = CreateFile(szVolumeAccessPath, GENERIC_READ | GENERIC_WRITE,
    
    ### Patches

 t/helper/test-drop-caches.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)