builtin/init-db: add environment variable for new repo hash

For the foreseeable future, SHA-1 will be the default algorithm for Git.
However, when running the testsuite, we want to be able to test an
arbitrary algorithm. It would be quite burdensome and very untidy to
have to specify the algorithm we'd like to test every time we
initialized a new repository somewhere in the testsuite, so add an
environment variable to allow us to specify the default hash algorithm
for Git.

This has the benefit that we can set it once for the entire testsuite
and not have to think about it. In the future, users can also use it to
set the default for their repositories if they would like to do so.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/init-db.c b/builtin/init-db.c
index d05552f..ab4fd68 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -20,6 +20,8 @@
 #define TEST_FILEMODE 1
 #endif
 
+#define GIT_DEFAULT_HASH_ENVIRONMENT "GIT_DEFAULT_HASH"
+
 static int init_is_bare_repository = 0;
 static int init_shared_repository = -1;
 static const char *init_db_template_dir;
@@ -356,6 +358,7 @@ static void separate_git_dir(const char *git_dir, const char *git_link)
 
 static void validate_hash_algorithm(struct repository_format *repo_fmt, int hash)
 {
+	const char *env = getenv(GIT_DEFAULT_HASH_ENVIRONMENT);
 	/*
 	 * If we already have an initialized repo, don't allow the user to
 	 * specify a different algorithm, as that could cause corruption.
@@ -365,6 +368,12 @@ static void validate_hash_algorithm(struct repository_format *repo_fmt, int hash
 		die(_("attempt to reinitialize repository with different hash"));
 	else if (hash != GIT_HASH_UNKNOWN)
 		repo_fmt->hash_algo = hash;
+	else if (env) {
+		int env_algo = hash_algo_by_name(env);
+		if (env_algo == GIT_HASH_UNKNOWN)
+			die(_("unknown hash algorithm '%s'"), env);
+		repo_fmt->hash_algo = env_algo;
+	}
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,