hash: add a constant for the default hash algorithm

Right now, SHA-1 is the default hash algorithm in Git.  However, this
may change in the future.

We have many places in our code that use the SHA-1 constant to indicate
the default hash if none is specified, but it will end up being more
practical to specify this explicitly and clearly using a constant for
whatever the default hash algorithm is.  Then, if we decide to change it
in the future, we can simply replace the constant representing the
default with a new value.

For these reasons, introduce GIT_HASH_DEFAULT to represent the default
hash algorithm.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

hash.h

@@ -174,6 +174,8 @@ static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *s
 #define GIT_HASH_SHA256 2
 /* Number of algorithms supported (including unknown). */
 #define GIT_HASH_NALGOS (GIT_HASH_SHA256 + 1)
+/* Default hash algorithm if unspecified. */
+#define GIT_HASH_DEFAULT GIT_HASH_SHA1
 
 /* "sha1", big-endian */
 #define GIT_SHA1_FORMAT_ID 0x73686131