We don't use "master" as a hardcoded default in as many places; remove
the now unused master branch constant.

When the remote does not tell us its default, we have to guess what the
default branch should be.  Use our local initial branch configuration to
inform the remote branch default when we clone.

We derive the branch name, even in our code, we shouldn't assume that
the branch will be "master".

When cloning an empty repository, we need to guess what the branch
structure should be; instead of hardcoding `master`, use the
`init.defaultBranch` setting it if it provided.

The git_repository_isempty function now respects the init.defaultbranch
setting (instead of hardcoding "master") to understand if a repository
is empty or not.

Provide a helper function to get the initial branch for a repository,
respecting the `init.defaultBranch` configuration option, if set, and
returning the "default default" (currently `master`) otherwise.

As part of a push towards more inclusive language, git is reconsidering
using "master" as the default branch name.  As a first step, this
setting will be configurable with the `init.defaultBranch` configuration
option.  Honor this during repository initialization.

During initialization, we will create an initial branch:

1. Using the `initial_head` setting, if specified;
2. Using the `HEAD` configured in a template, if it exists;
3. Using the `init.defaultBranch` configuration option, if it is set; or
4. Using `master` in the absence of additional configuration.

sanitizer ci: skip negotiate tests

We don't build with SPNEGO enabled on our focal-based sanitizer builds,
so we need to disable the negotiate tests.

Add CI support for Memory and UndefinedBehavior Sanitizers