Makefile: document default SHA-1 backend on OSX

Since [1] the default SHA-1 backend on OSX has been
APPLE_COMMON_CRYPTO. Per [2] we'll skip using it on anything older
than Mac OS X 10.4 "Tiger"[3].

When "DC_SHA1" was made the default in [4] this interaction between it
and APPLE_COMMON_CRYPTO seems to have been missed in. Ever since
DC_SHA1 was "made the default" we've still used Apple's CommonCrypto
instead of sha1collisiondetection on modern versions of Darwin and
OSX.

1. 6106795 (cache.h: eliminate SHA-1 deprecation warnings on Mac
   OS X, 2013-05-19)
2. 9c7a0be (config.mak.uname: set NO_APPLE_COMMON_CRYPTO on older
   systems, 2014-08-15)
3. We could probably drop "NO_APPLE_COMMON_CRYPTO", as nobody's likely
   to care about such on old version of OSX anymore. But let's leave that
   for now.
4. e6b07da (Makefile: make DC_SHA1 the default, 2017-03-17)

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: avar

Makefile

@@ -500,6 +500,11 @@ include shared.mak
 # Define BLK_SHA1 to make use of optimized C SHA-1 routines bundled
 # with git (in the block-sha1/ directory).
 #
+# Define NO_APPLE_COMMON_CRYPTO on OSX to opt-out of using the
+# "APPLE_COMMON_CRYPTO" backend for SHA-1, which is currently the
+# default on that OS. On macOS 01.4 (Tiger) or older,
+# NO_APPLE_COMMON_CRYPTO is defined by default.
+#
 # If don't enable any of the *_SHA1 settings in this section, Git will
 # default to its built-in sha1collisiondetection library, which is a
 # collision-detecting sha1 This is slower, but may detect attempted