Fixes #1724.

This patch omits the "Signed-off-by:" line coming from pull requests created
out of single commit messages. This way the user does not need to manually
remove their email address from the PR description.

Not introducing a flag for this as it applies to only one case (single-commit
diff between heads, as well as no -m/-f/-i specified).

Meta: sending this PR with the patched build, my Signed-off-by line is removed
automatically from the PR message.
Signed-off-by: NAhmet Alp Balkan <ahmetb@google.com>

Closes #1740

Add milestone to PR by name instead of number

[RFC] Make fork --remote-name=origin rename origin

If user passes `origin` as the remote name
pointing to forked repository, it's likely
she wants to rename origin as upstream.

[ci skip]

Add install instructions for scoop on Windows

When querying `foo/bar` repo redirects elsewhere, assume that `foo/bar`
is available. Fixes #1685

Fixes #1549

Document sync command in help

CONTRIBUTING: note license.

This is to be more explicit about the contribution process and license.

The default colored output isn't very useful when labels stand on their own.

Forgot to change expected result after changing labels functionality.

Add `hub pr list` command to list pull requests

Merging is for consistency of organization, since 'labels' is a
sub-command of 'issue', as is 'create', which is also inside
commands/issue.go.

Extract getSuitableTextColor in commands/issue.go as a result of
merger.

(based off commands/issue.go)

'hub issue labels' makes more semantic sense to retrieve all
available labels from the issue tracker. The command 'label'
sounds like the imperative form, i.e. to label an issue.