narrow: mark wire proto capability names experimental and versioned
We already plan to add a "widen" wire protocol command to the "narrow"
capability, so let's version the capabilities as "exp-narrow-1" and
"exp-ellipses-1". When we add the "widen" command, we will then add a
"exp-narrow-2" capability to indicate support for that command.
Differential Revision: https://phab.mercurial-scm.org/D4529
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