Pkgdev

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- pkgdev commit: Add all matching pkg versions to historical repo (40).

- pkgdev commit: Use ``git diff-index`` instead of ``git diff`` to avoid config
settings affecting output.

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- pkgdev showkw: Add bash completion support (38).

- pkgdev commit: Generate summaries for package changes with profile updates,
e.g. renaming a package and updating profiles/updates in the same commit.

- pkgdev commit: Avoid crash when footer content exists with no summary
template (39).

- pkgdev commit: Add initial support for generating summaries from bash diffs.
For example, this allows automatic summaries to be generated for simple
PYTHON_COMPAT changes.

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- Fix historical repo creation for eclass sourcing.

- Add initial bash completion support.

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- pkgdev show: Analog to eshowkw from gentoolkit migrated from pkgcore's
pshowkw.

- pkgdev manifest: Add -d/--distdir option for custom DISTDIR.

- pkgdev mask: Change removal format to a 'tag: value' style.

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- pkgdev mask: Initial implementation of package.mask mangling support.

- pkgdev commit: Allow -s/--scan to accept an optional boolean arg for
consistency.

- pkgdev commit: Support partial package manifesting (33).

- pkgdev commit: Add -T/--tag option to add generic commit tags.

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- pkgdev commit: Support pulling historical data from unconfigured repos.

- Add initial zsh completion support (16).