Document how the default branch name can be overridden

There is a `GIT_TEST_*` environment variable and a `core.` config
setting (with the former taking precendence over the latter) to allow
overriding what name Git uses by default as main branch of new
repositories.

Now that all kinds of Git operations have learned to respect those,
let's document them.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

Documentation/config/core.txt

@@ -626,3 +626,7 @@ core.abbrev::
 	in your repository, which hopefully is enough for
 	abbreviated object names to stay unique for some time.
 	The minimum length is 4.
+
+core.defaultBranchName::
+	Allows overriding the default branch name e.g. when initializing
+	a new repository or when cloning an empty repository.

t/README

@@ -421,6 +421,10 @@ GIT_TEST_DISALLOW_ABBREVIATED_OPTIONS=<boolean>, when true (which is
 the default when running tests), errors out when an abbreviated option
 is used.
 
+GIT_TEST_DEFAULT_BRANCH_NAME allows overriding the default branch name
+that is used for example when initializing new repositories, or when
+cloning a repository that has no branches yet.
+
 Naming Tests
 ------------