Currently, we only provide functions to read references directly from a
repository's reference store via e.g. `git_reference_lookup`. But in
some cases, we may want to read files not connected to the current
repository, e.g. when looking up HEAD of connected work trees. This
commit implements `git_reference__read_head`, which will read out and
allocate a reference at an arbitrary path.

Fixups requested in #3912.

Introduce GIT_OPT_ENABLE_SYMBOLIC_REF_TARGET_VALIDATION option.
Setting this option to 0 allows
validation of a symbolic ref's target to be bypassed.
This option is enabled by default.

This mechanism is added primarily to address a discrepancy between git
behaviour and libgit2 behaviour, whereby the former allows the symbolic
ref target to carry an arbitrary string and the latter does not, so:

    $ git symbolic-ref refs/heads/foo bar
    $ cat .git/refs/heads/foo
    ref: bar

where as attempting the same via libgit2 raises an error:

    The given reference name 'bar' is not valid

this mechanism also allows those that might want to make use of
git's more lenient treatment of symbolic ref targets to do so.

These are not quite like their plain counterparts and require special handling.

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 transaction allows you to lock multiple references and set up changes
for them before applying the changes all at once (or as close as the
backend supports).

This can be used for replication purposes, or for making sure some
operations run when the reference is locked and thus cannot be changed.