Add xdelta as dependency to snapcraft formula

[mgrachev]

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Hi there!

When I tried to push a snap package via snapcraft, I got the error:

A tool snapcraft depends on could not be found: 'xdelta3'.
Ensure the tool is installed and available, and try again.
Sorry, an error occurred in Snapcraft:
Traceback (most recent call last):
  File "/usr/local/bin/snapcraft", line 11, in <module>
    load_entry_point('snapcraft==3.0.1', 'console_scripts', 'snapcraft')()
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/cli/__main__.py", line 81, in run
    run_snapcraft(prog_name="snapcraft")
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/cli/store.py", line 161, in push
    snapcraft.push(snap_file, channel_list)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/_store.py", line 552, in push
    result = _push_delta(snap_name, snap_filename, source_snap)
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/_store.py", line 595, in _push_delta
    source_path=source_snap, target_path=target_snap
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/internal/deltas/_xdelta3.py", line 28, in __init__
    delta_tool_path = file_utils.get_tool_path("xdelta3")
  File "/usr/local/Cellar/snapcraft/3.0.1/libexec/lib/python3.7/site-packages/snapcraft/file_utils.py", line 355, in get_tool_path
    raise ToolMissingError(command_name=command_name)
snapcraft.internal.errors.ToolMissingError: A tool snapcraft depends on could not be found: 'xdelta3'.

So, I added xdelta as a dependency to the snapcraft formula.

[fxcoudert]

Thanks @mgrachev