Enable SHA-256 by default in breaking changes mode

Our document on breaking changes indicates that we intend to default to
SHA-256 in Git 3.0.  Since most people choose the default option, this
is an important security upgrade to our defaults.

To allow people to test this case, when WITH_BREAKING_CHANGES is set in
the configuration, build Git with SHA-256 as the default hash.  Update
the testsuite to use the build options information to automatically
choose the right value.

Note that if the command substitution for GIT_TEST_BUILTIN_HASH fails,
so does the testsuite—and quite spectacularly at that.  Thus, the case
where the Git binary is somehow subtly broken will not go undetected.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hash.h

@@ -174,8 +174,14 @@ 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
+#ifdef WITH_BREAKING_CHANGES
+# define GIT_HASH_DEFAULT GIT_HASH_SHA256
+#else
+# define GIT_HASH_DEFAULT GIT_HASH_SHA1
+#endif
+
 /* Legacy hash algorithm. Implied for older data formats which don't specify. */
 #define GIT_HASH_SHA1_LEGACY GIT_HASH_SHA1
 

t/test-lib.sh

@@ -536,7 +536,7 @@ export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
 export EDITOR
 
-GIT_TEST_BUILTIN_HASH=sha1
+GIT_TEST_BUILTIN_HASH=$("$GIT_BUILD_DIR/git" version --build-options | sed -ne 's/^default-hash: //p')
 GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-$GIT_TEST_BUILTIN_HASH}"
 export GIT_DEFAULT_HASH
 GIT_DEFAULT_REF_FORMAT="${GIT_TEST_DEFAULT_REF_FORMAT:-files}"