Crash and unclear error message when pushing without a commit

[mhlakhani]

I hit this crash / error message today, after running (effectively) the following:

sl init --git myrepo
cd myrepo
# create a bunch of files
sl add .
# I forgot to commit...
sl path --add remote/default ssh://git@github.com/mhlakhani/myrepo.git
sl push --to remote/default

This resulted in:

To ssh://github.com/mhlakhani/myrepo.git
 - [deleted]         default
** Sapling SCM (version 0.2.20231113-145254+995db0d6) has crashed:
Traceback (most recent call last):
  File "static:sapling", line 86, in run
    dispatch.run(args, fin, fout, ferr, config)
  File "static:sapling.dispatch", line 160, in run
    status = (dispatch(req) or 0) & 255
              ^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 377, in dispatch
    ret = _runcatch(req)
          ^^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 561, in _runcatch
    return _callcatch(ui, req, _runcatchfunc)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 570, in _callcatch
    return scmutil.callcatch(ui, req, func)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.scmutil", line 153, in callcatch
    return func()
           ^^^^^^
  File "static:sapling.dispatch", line 548, in _runcatchfunc
    return _dispatch(req)
           ^^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 1078, in _dispatch
    ret = runcommand(
          ^^^^^^^^^^^
  File "static:sapling.ext.undo", line 176, in _runcommandwrapper
    result = orig(lui, repo, cmd, fullargs, *args)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.ext.undo", line 120, in origunsetpid
    return _orig(*args, **opts)
           ^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.ext.journal", line 80, in runcommand
    return orig(lui, repo, cmd, fullargs, *args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.ext.sparse", line 444, in _tracktelemetry
    res = runcommand(lui, repo, *args)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.ext.copytrace", line 191, in _runcommand
    return orig(lui, repo, cmd, fullargs, ui, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 765, in runcommand
    ret = _runcommand(ui, options, cmd, d)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.dispatch", line 1119, in _runcommand
    return cmdfunc()
           ^^^^^^^^^
  File "static:sapling.dispatch", line 1077, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **strcmdopt)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.util", line 1396, in check
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.util", line 1396, in check
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.ext.remotenames", line 876, in expushcmd
    return git.push(repo, dest, pushnode, to, force=force)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.git", line 615, in push
    with repo.lock(), repo.transaction("push"):
  File "static:sapling.util", line 772, in __exit__
    self.close()
  File "static:sapling.transaction", line 71, in _active
    return func(self, *args, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "static:sapling.transaction", line 579, in close
    self.after()
  File "static:sapling.localrepo", line 3309, in a
    repo.changelog.inner.updatereferences(repo.metalog())
error.UncategorizedNativeError: target OID for the reference doesn't exist on the repository; class=Reference (4)
Exception ignored in: <function transaction.__del__ at 0x1092aa8e0>
Traceback (most recent call last):
  File "static:sapling.transaction", line 253, in __del__
  File "static:sapling.transaction", line 696, in _abort
  File "static:sapling.vfs", line 314, in unlink
FileNotFoundError: [Errno 2] No such file or directory: '/Users/mhl/projects/myrepo/.sl/store/journal'

The solution was pretty simple: sl commit -m "something"

Could we detect this (admittedly uncommon...) case and avoid a crash/improve the error message?