Make symbolic ref target validation optional

Make symbolic ref target validation optional

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.