init: allow setting the default for the initial branch name via the config

init: allow setting the default for the initial branch name via the config

We just introduced the command-line option
`--initial-branch=<branch-name>` to allow initializing a new repository
with a different initial branch than the hard-coded one.

To allow users to override the initial branch name more permanently
(i.e. without having to specify the name manually for each and every
`git init` invocation), let's introduce the `init.defaultBranch` config
setting.

Helped-by: Johannes Schindelin <redacted>
Helped-by: Derrick Stolee <redacted>
Signed-off-by: Don Goodman-Wilson <redacted>
Signed-off-by: Junio C Hamano <redacted>