copies: document how 'copies' dict instances are reused
We avoid copying these instances as much as we can, so it's not
obvious what's safe to do with them. This patch tries to explain what
is safe and what is not.
Differential Revision: https://phab.mercurial-scm.org/D6578
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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