manifest: change manifestctx to not inherit from manifestdict
If manifestctx inherits from manifestdict, it requires some weird logic to
lazily load the dict if a piece of information is asked for. This ended up being
complicated and unintuitive to use.
Let's move the dict creation to .read(). This will make even more sense once we
start adding readdelta() and other similar methods to manifestctx.
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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