Makefile & test-tool: replace "DC_SHA1" variable with a "define"

Address the root cause of technical debt we've been carrying since
sha1collisiondetection was made the default in [1]. In a preceding
commit we narrowly fixed a bug where the "DC_SHA1" variable would be
unset (in combination with "NO_APPLE_COMMON_CRYPTO=" on OSX), even
though we had the sha1collisiondetection library enabled.

But the only reason we needed to have such a user-exposed knob went
away with [1], and it's been doing nothing useful since then. We don't
care if you define DC_SHA1=*, we only care that you don't ask for any
other SHA-1 implementation. If it turns out that you didn't, we'll use
sha1collisiondetection, whether you had "DC_SHA1" set or not.

As a result of this being confusing we had e.g. [2] for cmake and the
recent [3] for ci/lib.sh setting "DC_SHA1" explicitly, even though
this was always a NOOP.

A much simpler way to do this is to stop having the Makefile and
CMakeLists.txt set "DC_SHA1" to be picked up by the test-lib.sh, let's
instead add a trivial "test-tool sha1-is-sha1dc". It returns zero if
we're using sha1collisiondetection, non-zero otherwise.

1. e6b07da278 (Makefile: make DC_SHA1 the default, 2017-03-17)
2. c4b2f41b5f (cmake: support for testing git with ctest, 2020-06-26)
3. 1ad5c3df35 (ci: use DC_SHA1=YesPlease on osx-clang job for CI,
   2022-10-20)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

sha1dc_git.h

@@ -17,6 +17,7 @@ void git_SHA1DCInit(SHA1_CTX *);
 void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
 void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
 
+#define platform_SHA_IS_SHA1DC /* used by "test-tool sha1-is-sha1dc" */
 #define platform_SHA_CTX SHA1_CTX
 #define platform_SHA1_Init git_SHA1DCInit
 #define platform_SHA1_Update git_SHA1DCUpdate