Stacked Git 1.2

This release is choc full of new features, including
stg rebase --interactive and stg rebase --autostash as well as many
quality of life improvements to new, edit, squash.

StGit v1.2 updates the stack metadata. Gone are .stgit branches!
Instead, stack metadata refs are now found in the new refs/stacks
namespace instead of the regular branches namespace (refs/heads). This
change eliminates clutter when looking at branch lists and in various
forms of git log.

An important packaging change to be aware of is that StGit now uses
setuptools instead of the deprecated distutils. This change will affect
downstream StGit packagers, but hopefully StGit vectoring toward modern
Python packaging standards will be a positive in the long run.

Many thanks to Topher Brown for significant contributions to StGit
v1.2!

Deprecated

• Python 3.5, which became EOL 2020-09-13, support is deprecated and
  will be removed in a future StGit release
• Python 3.6, which will be EOL 2021-12-23, support is deprecated and
  will be removed in a future StGit release

Added

• stg rebase learns --interactive; easily re-order, edit, squash,
  fixup, or delete patches via your editor
• stg rebase learns --autostash; stash changes before the rebase and
  apply them after. Also configurable with the stgit.autostash
  configuration option
• stg edit can now rename patches (#119)
• stg edit gains helpful instructions (#138)
• stg new learns --verbose, which includes a diff in the editor
  window (similar to git commit --verbose). This behavior is also
  configurable with the stgit.new.verbose configuration option
• stg push and stg float learn --noapply option; allows patches
  to be reordered without updating worktree and deferring merge conflict
  resolution (#144)
• stg edit, stg refresh, and stg new learn the --sign-by,
  --ack-by, and --review-by options which allow those respective
  trailers' values to be specified by the user on the command line (#92)

Changed

• Stack metadata version 5; stack metadata is moved from
  refs/heads/<branch>.stgit to refs/stacks/<branch> and the stack
  metadata file now uses a JSON format instead of the prior custom
  format; the stack metadata will be upgraded to v5 on first use of
  this version of StGit; like all stack metadata upgrades, this is a
  one-way auto-upgrade for existing stacks (#65)
• Use setuptools instead of distutils for packaging
• No git or python version checks in setup.py
• Use different dynamic versioning system
• Install stg executable as console_script entry point
• More sophisticated search for bash.exe on Windows when running hooks
• The editor window text for stg squash has been modified to mirror
  git's behavior -- the squash edit message now includes all commits
  (#71)
• Binary diffs are no longer shown when with stg edit -d
• Multiple trailers can now be added at once; this is now allowed, for
  example: stg edit --sign --review --ack
• Update zsh completion for stg rebase to show local and remote heads
  (#102)
• Zsh completions for commands with patch arguments now comprehend the
  effect of -b/--branch and -B/--ref-branch
• Zsh completions now guard patch names--one less TAB press to complete
  patch names in certain contexts
• stg import now extracts the Message-ID email header into the patch
  message (#42)

Fixed

• Repair crash when attempting to export empty patch (#112)
• Exact command name matches are unambiguous (#110)
• Exiting with an empty stg edit editor will now abort the edit;
  previously it would delete your commit message. (#138)
• Repair completions when stg.series.description is enabled in config
• Workaround child process reaping race on Windows (#78)
• Repair crash with stg float --series when bad patch name in series
• Repair zsh completion for stg float to accept multiple patch names
• Repair zsh completion for changed files, affecting stg refresh and
  stg diff

Internal

• Add link to coverage.io project to CONTRIBUTING.md
• Set smart exclude_lines default for 'coverage'
• Expanded test suite for stg edit
• Add pkgtest.py script to help test StGit packaging
• Cleanup .gitignore files