testsvn: respect `init.defaultBranch`

testsvn: respect `init.defaultBranch`

The default name of the initial branch in new repositories can now be
configured. The `testsvn` remote helper translates the remote Subversion
repository's branch name `trunk` to the hard-coded name `master`.
Clearly, the intention was to make the name align with Git's defaults.

So while we are not talking about a newly-created repository in the
`testsvn` context, it is a newly-created _Git_ repository, si it _still_
makes sense to use the overridden default name for the initial branch
whenever users configured it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>