Fix for TypeError in Repository.ident (plus typo fixes)

[rapidcow]

As of pygit2 1.19.2 (on my Python 3.14.4, Linux 6.19.14-artix1-1 x86_64), the Repository.ident property triggers TypeError:

Python 3.14.4 (main, Apr 15 2026, 07:34:47) [GCC 15.2.1 20260209] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pygit2; pygit2.__version__
'1.19.2'
>>> repo = pygit2.init_repository('/tmp/foo')
>>> repo.ident
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    repo.ident
  File "/usr/lib/python3.14/site-packages/pygit2/repository.py", line 1588, in ident
    return (ffi.string(cname).decode('utf-8'), ffi.string(cemail).decode('utf-8'))
            ~~~~~~~~~~^^^^^^^
TypeError: string(): unexpected cdata 'char * *' argument

cname and cemail are char ** variables, which are pointers to C-strings, not the C-strings themselves. So they should be passed as cname[0] and cemail[0].

In addition, cname[0] and cemail[0] appear to be NULL by default... so clearly we have to handle that separately.

Either way, I wrote up a patch (plus a typo I happened to catch when trying to skim the docs for other mentions of identity). Not sure if this is the right way to contribute here... any feedback* is appreciated! :)

*Specifically, (1) I'm not certain if utils.maybe_string() is the right tool for this, since the original UTF-8 decode mode was "strict", but is now "surrogateescape" (but I don't want to introduce a separate function just for this, so...) Also (2) I used PEP 585 syntax for the tuple annotation, while keeping Optional since it has already been imported (pygit2 itself seems to use a mix of pre- and post-PEP 585 syntax, and ruff doesn't appear to complain either way, so I picked the ones I liked the best without importing more names...)

[rapidcow]

The commit wording was a bit bad, sorry...

# git range-diff @{u}~3..@{u} @~3..@
1:  ea5b51b7 ! 1:  d32573ec fix(repository): dereference reflog identity correctly
    @@ Commit message
         as C-strings, when they really are pointers to C-strings.
         Fix by dereferencing the pointer to a C-string first.
     
    -    Furthermore, Repository.ident assumes that libgit2 holds
    -    a valid C-string, when the reflog identity is initialized
    +    Furthermore, Repository.ident assumes that said variables
    +    hold valid C-strings, when the reflog identity is initialized
         to NULL, and can be explicitly set to such with `set_ident
         (None, None)`.  So map None to ffi.NULL (which maybe_string()
         happens to already do, so I simply delegated to that...).
2:  9b65c2d6 = 2:  379b4d82 fix(repository): amend annotations on reflog identity
3:  1deb18ba = 3:  e7bd6d26 docs(repository): fix default_signature typos