This also brings the soft-reset tests back to life. The function name
was missing an underscore, meaning they had not been running.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

A repository can have multiple "reserved names" now, not just
a single "short name" for the repository folder itself.  Refactor
to include a git_repository__reserved_names that returns all the
reserved names for a repository.

Introduce GITTEST_INVASIVE_FS_STRUCTURE for things that are invasive
to your filesystem structure (like creating folders at your filesystem
root) and GITTEST_INVASIVE_FS_SIZE for things that write lots of data.

Checkout performance

Test to ensure that we can create a repository at the filesystem
root.  Introduces a new test environment variable,
`GITTEST_INVASIVE_FILESYSTEM` for tests that do terrible things like
escaping the clar sandbox and writing to the root directory.  It is
expected that the CI builds will enable this but that normal people
would not want this.