cache-tree/blame: avoid reusing the DEBUG constant

cache-tree/blame: avoid reusing the DEBUG constant

In MS Visual C, the `DEBUG` constant is set automatically whenever
compiling with debug information.

This is clearly not what was intended in `cache-tree.c` nor in
`builtin/blame.c`, so let's use a less ambiguous name there.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>