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.

If we cannot dwim the input, set the error message to be explicit about
that. Otherwise we leave the error for the last failed lookup, which
can be rather unexpected as it mentions a remote when the user thought
they were trying to look up a branch.

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.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

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.

This fixes two issues I found when core.precomposeunicode is enabled:

* When creating a reference with a NFD string, the returned
  git_reference would return this NFD string as the reference’s
  name. But when looking up the reference later, the name would
  then be returned as NFC string.
* Renaming a reference would not honor the core.precomposeunicode and
  apply no normalization to the new reference name.

Keep the reflog parameters as the last two, as they're the optional
parameters.

Add it under the git_reference_remove() name, letting the user pass the
repo and name, analogous to unconditional setting/creation.

Recognize when the reference has changed since we loaded it.

We don't actually pass the old value yet.

Bring the race detection goodness to symbolic references as well.

Add a parameter to the backend to allow checking for the old symbolic
target.

Change the name to _matching() intead of _if(), and force _set_target()
to be a conditional update. If the user doesn't care about the old
value, they should use git_reference_create().

Allow updating references if the old value matches the given one.

Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.

Signed-off-by: Brodie Rao <brodie@sf.io>

It's exported in the headers, but the implementation was missing.