wireprotov2: change name and behavior of readframe()
In the near future, we will want to support performing I/O from
other sources. Let's rename readframe() to readdata() and tweak
its logic to support future growth.
Differential Revision: https://phab.mercurial-scm.org/D4771
Mercurial
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Mercurial is a fast, easy to use, distributed revision control tool
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