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.

Pass file descriptors to cmd.NewWithArray

Fix windows build

Add plan9port rc shell support