Gitless

- upgrade libgit2 to 0.28 so that the gitless homebrew formula can be updated (thanks chenrui333 for reporting the problem)

The Mac OS binary was packaged with Python 2.7.16, libgit2 0.28.1, and PyInstaller 3.4 on Mac OS 10.14.5

- pin pygit2 version to avoid breakages (thanks techtonik)
- catch potential decode errors on stdout/stderr and fix typo (thanks breisfeld)
- fix pygit2 0.27 breaking change (`pygit2.discover_repository()` returns `None`) (thanks ppentchev)
- snap packaging (thanks abitrolly)
- upgrade py/libgit2 to 0.27

The Mac OS binary was packaged with Python 2.7.16 and PyInstaller 3.4 on Mac OS 10.14.15

- more info on `gl commit` and `gl publish` output
- use the branch name in the `to create a new branch do gl branch -c` help message of `gl switch` (thanks to katrinleinweber)
- relicense from GPLv2 to MIT (thanks to all the contributors who replied to my email)
- improve help message (`gl -h`) (thanks to techtonik)
- remove `-o` option from `gl diff` and `gl commit`. To specify a list of files to commit simply list them (i.e., instead of `gl diff -o foo` do `gl diff foo`) (thanks to techtonik)
- remove the using-upstream-branch warning on `gl publish` (thanks to dogweather)
- `gl checkout` now does a recursive checkout of all files under a directory if a directory is given as input
- internal: fix travis and appveyor (thanks to techtonik, embs)
- add testing instructions to readme (thanks to embs)

- windows support
- only page output if we are running in a tty
- skip ignored directories when processing files
- `gl` without any argument now displays the help message
- group flags of `gl branch` and `gl tag` and throw error on invalid combinations
- sort the list of tags and branches when listed
- add windows pager (thanks to ArneBachmann)
- other pager-related improvements (thanks to ppentchev, raimue )
- improvements to readme: link pygit2 for Windows users (thanks to abitrolly ), svg badges (thanks to sobolevn), add homebrew badge (thanks to emartynov)

- show error if there's no commit author set
- show error if we can't launch editor
- cover more values for the `color.ui` setting (thanks to vmx)
- fixed potential unicode encoding errors (thanks to knowsuchagency)
- changed reqs declaration to use current version of pygit2 (thanks to scottsideleau)
- other misc fixes: updated link to google python style guide on readme (thanks to rdpate), include full copy of GPLv2

- new `gl tag` command to list, create, and delete local and remote tags
- new `-p/--partial` flag in `gl commit` to interactively select segments of files to commit
- fixed a bug that caused Gitless to think it was still in the middle of a merge after an unsuccessful application of uncommitted saved changes