util: set correct stack level on deprecation warnings
Without this patch, you'll get something like this:
<path>/mercurial/util.py:3784: DeprecationWarning: 'util.hgexecutable'
is deprecated, use 'utils.procutil.hgexecutable'
(but on one line)
Differential Revision: https://phab.mercurial-scm.org/D3331
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