Change the default branch name to main

The current default name for the initial branch is a loaded term, and
many Open Source projects renamed their principal branches already. A
common choice appears to be `main`.

Let's follow their lead and change the default of `init.defaultBranch`.

Co-authored-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Don Goodman-Wilson <don@goodman-wilson.com>

refs.c

@@ -575,7 +575,7 @@ char *repo_default_branch_name(struct repository *r)
 		die(_("could not retrieve `%s`"), config_display_key);
 
 	if (!ret)
-		ret = xstrdup("master");
+		ret = xstrdup("main");
 
 	full_ref = xstrfmt("refs/heads/%s", ret);
 	if (check_refname_format(full_ref, 0))

t/lib-submodule-update.sh

@@ -144,7 +144,7 @@ create_lib_submodule_repo () {
 		git checkout -b valid_sub1 &&
 		git revert HEAD &&
 
-		git checkout "${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME-master}"
+		git checkout "${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME-main}"
 	)
 }