Be sure to clean up looked up references.  Free buffers instead of
merely clearing them.  Use `git__free` instead of `free`.

Whenever we rename a branch, we update the repository's symbolic HEAD
reference if it currently points to the branch that is to be renamed.
But with the introduction of worktrees, we also have to iterate over all
HEADs of linked worktrees to adjust them. Do so.

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.

This fixes issue #4094

Fixups requested in #3912.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

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.