In `hub clone NAME`, "NAME" was previously considered a local resource
if a directory of the same name existed, and the clone command was left
unchanged.

That worked for a while, but some users were surprised that they
couldn't clone their repo named "NAME", only to discover that there was
an unrelated directory "NAME" in the current working directory.

Also, a git bundle is a valid cloneable resource, but is a file and not
a directory.

This refines the detection of cloneable resources. "NAME" is considered
to be locally cloneable if one of the following is true:

- "NAME/.git" exists
- "NAME" is a bare git repo
- "NAME" is a file whose first line includes "git bundle"

To obtain the owner/fork-name combo, instead of reading it from "label"
field from the PR response simply read from `Head.Ref` and
`Head.Repo.Owner.Name` instead. This is because label is meant to be
human-readable, isn't well-documented in GitHub API docs and is unclear
when it contains two components (delimited by `:`) and when it contains
only one.

`bufio.Scanner` takes care of line endings difference for different OSes. See http://golang.org/pkg/bufio/#ScanLines.

This fixes #805

This reverts commit 418c71d4. See
discussion https://github.com/github/hub/pull/550#issuecomment-40901808

When creating pull request with `-m` flag in the command line, "\n" appears as "\\n",
failing to split title and body. See https://github.com/jingweno/gh/issues/200#issuecomment-38517724

Since https://github.com/github/hub/compare/31a85e519e0a080dd10ce21339f806a9a0877766...master

Some tests are ignored either because they don't make sense or WIP

Removes duplicated test helper.