Stacked Git 1.0

The 1.0 release has been over a decade in the making. While light on new
features, this release represents a major upgrade in StGit's foundation.

Internally, StGit now maintains all metadata in Git objects. This
enables novel workflows involving StGit stacks to be pushed and pulled
from remote repositories using git push and git pull.

And StGit has been fully migrated to Python 3. Dropping compatibility
with Python 2 has allowed StGit's code to become simpler and avoid
error-prone compatibility code. Python 3.5 is the new minimum required
version.

Removed

• Drop support for Python < 3.5
• Remove previously deprecated stg publish command
• Removed contrib scripts: stg-swallow, stg-fold-files-from,
  stg-dispatch, stg-whatchanged, and stg-show-old

Added

• The pre-commit hook is now run for stg refresh
• New --spill option for stg refresh
• Add stgit.series.description config option (#88)
• Official support for Python versions up to 3.9

Changed

• Stack metadata format 4. All metadata now kept in Git objects; no more
  stack state files in .git/patches. A one-way auto-upgrade to format
  version 4 will occur when StGit commands are run on an existing StGit
  branch.
• Use python3 in shebangs instead of python
• contrib/stgbashprompt.sh is no longer executable
• Internal docstrings now use reStructuredText instead of Epytext

Fixed

• Importing large patches is much, much faster (#66)
• Other performance improvements when dealing with large patches
• Repair diffstat when outside work tree root (#62)
• Use encoded (string) environment variables on Windows (#79)
• Fix stg pull when no upstream is configured (#83)
• Fix refresh crash with path limiting and files added to index (#85)
• Repair new with patchdescr.template crash (#87)
• Repair log from worktree subdir with patches specified
• Repair import allowing/generating duplicate patch names (#64)
• Repair mail --auto to strip comments after addrs (#91)