t6300: avoid using the default name of the initial branch

Our test suite currently only passes when `git init` uses the name
`master` for the initial branch. This would stop us from changing the
default branch name.

Let's adjust t6300 so that it does not rely on any specific default
branch name. This trick is done by (force-)renaming the initial branch
to the name `main` in the `setup` and the `:remotename and :remoteref`
test cases, and then replacing all mentions of `master` and `MASTER`
with `main` and `MAIN`, respectively.

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

Author: dscho