Add support for naming release and builds accordingly when given a git
ref for a pretest (90 or above patch number) or release
candidate ("-rcX" at the end of the tag).
Stable builds are based off of release git tags in Emacs' git
repo. Examples of what release tags look like:
- emacs-26.1
- emacs-26.2
- emacs-26.3
- emacs-27.1
- emacs-27.2
When the specified git ref looks like a stable release, the plan command
will generate a release a different and simpler release name that does
not include the date, git sha or ref. Instead, for "emacs-27.2" for
example, the emacs-builds release name will be "Emacs-27.2".
The "build name", used for naming the disk image, still retains the same
format as the nightly builds.
Also, non-stable releases are now marked as pre-release on GitHub by
default.
The reason for the different release name format for stable builds is
both to separate them, but also to make it easier to keep the version of
the homebrew cask as simply "27.2".
