Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Remy Suen <remy.suen@gmail.com>

As git_reference__name will reallocate storage to account for longer
names (it's actually allocator-dependent), it will cause all existing
pointers to the old object to become dangling, as they now point to
freed memory.

Fix the issue by renaming to a more descriptive name, and pass a pointer
to the actual reference that can safely be invalidated if the realloc
succeeds.

Implement a new function that is able to determine if a branch is
checked out in any repository connected to the current
repository. In particular, this is required to check if for a
given repository and branch, there exists any working tree
connected to that repository that is referencing this branch.

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 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.