When we look for which remote corresponds to a remote-tracking branch,
we look in the refspecs to see which ones matches. If none do, we should
abort. We currently ignore the error message from this operation, so
let's not do that anymore.

As part of the test we're writing, let's test for the expected behaviour
if we cannot find a refspec which tells us what the remote-tracking
branch for a remote would look like.

We do not always want to put the id directly into the reflog, but we
want to speicfy what a user typed. For this use-case we provide
annotated version of a few functions which let the caller specify what
user-friendly name was used when asking for the operation.

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.

This namespace is about behaving like git's branch command, so let's do
exactly that instead of taking a reflog message.

This override is still available via the reference namespace.

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.

We want to ignore GIT_ENOTFOUND, but for that we need to capture the
error code from the reflog deletion.