For compatibility with git, `hub help checkout` will display the help
for git-checkout and there will be no information about hub extensions
to this command.

Now with `hub help hub-checkout`, hub will print the help text from
hub's extension to git-checkout.

- No more `c.Short` property. Instead, the first line of `c.Long`
  property is considered a short command description.

- The `c.Usage` text can now contain multiple lines.

- The new `c.Synopsis()` method renders usage synopsis for humans:

    Usage: hub my-command --arg
           hub my-command --alternative-arg

- The new `c.HelpText()` method renders synopsis + full help text.

Don't corrupt the PATH if longer than 1024 characters on Windows

Help text improvements

Improve detecting local clone resources

Fixes #862

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"

Add new flag to "release" for specifying commitish

Avoid "user: Current not implemented" error

Detect current username without resorting to cgo

Sometimes when hub is cross-compiled, `user.Current()` won't work on
certain systems. Use an alternative way of obtaining the user's home
directory.

- git+ssh://github.com/...
- ssh://github.com:22/...
- git@github.com:/...

Fixes #876

In instances where the `SHELL` environment variable is not set, nor is the `-s` parameter set it is not clear what action should occur.  Clarify the error message to make it obvious.

Closes #1065

Untested.

Fixes #896

Fixes #1057

rename gh command to hub in docs

document that clone directory is optional

setx will automatically truncate the PATH to 1024 characters if it is longer than that. It also mixes the local user's path with the machine-wide path, which is no good.

Unfortunately, the only way around this is running PowerShell to set the PATH manually.

Related link: http://superuser.com/questions/387619/overcoming-the-1024-character-limit-with-setx