Have `fetch` use writable push URL for writeable fork

This is to avoid having a writeable fork added with `git://` protocol
(the default when `hub.protocol` is not configured).

This matches the behavior of `hub clone`.

Enable team reviewers in `pull-request`

Spelling

Fix recognition logic for vi editor variants again

Ensure that _git completion func is replaced only once.

This allows the completion script to be sourced more than once while
still behaving correctly. Previously, the script would attempt to
replace `__git_list_all_commands` blindly, which would yield
multiply-replaced calls like
`__git_list_all_commands_without_hub_without_hub`.

Fix recognition logic for vi editor variants

Fixes #1602

Fix tmux_pane_content test helper

Fetch all releases when doing `hub release`

Fixes #1511

Fix PATH update in windows installation script

Fixes #1506.

Ignore all lines that are printed before the first command prompt. On
some systems bash prints a message about how to use the sudo command
when started with a clean `HOME`, causing some tests to fail.

Fixes #1477

As suggested at #1511, instead of fetching only the first 30 releases,
follow the `next` link to fetch all pages.

Document available environment variables

travis: update go versions

As discussed at https://github.com/github/hub/pull/1573

Enforce unix eol in bash script files through .gitattributes

Contributor Covenant Code of Conduct

Document the availability of the development version.