Fixes #896

Fixes #1057

rename gh command to hub in docs

document that clone directory is optional

commands/fork: handle missing "origin" remote

Fixes nil reference crash. Fixes #1042.

Cache result of `git rev-parse --git-dir`

Another attempt at fixing pull-request command for fetch-only remotes.

References #876 

It is a common practice to set the push URL of the "upstream" repository to an
invalid URL to avoid accidentally pushing to upstream instead of a fork. The
trouble with the current approach of obtaining remote URLs is that, the URL
obtained for a remote might not be the valid one, even though a valid remote
fetch URL exists. For example, git remote -v might return the following output

~$ git remote -v

origin      https://github.com/owner/project.git (fetch)
origin      https://github.com/owner/project.git (push)
upstream    https://github.com/main/project.git (fetch)
upstream    no_push (push)

But github.Remotes() currently parses both these URLs but stores only one of
them, the one that comes the last in git remote -v output, for a given remote.
So in this example, the URL for upstream remote might end up to be "no_push"
instead of https://github.com/main/project.git. This causes problems
downstream. For example, pull-request can't proceed with non github.com URLs
and the command fails even when a valid fetch URL for upstream is defined.

This commit fixes the problem by parsing and storing both fetch and push URLs
for each remote. So later, wherever the URLs are required, we can either
look at fetch URL or push URL or both depending on what is required.

Fixes one part of issue #876. Still doesn't recognize "git+ssh" protocol
scheme.

Also, define AddRemote() method for test Repo fixture.

It's invoked often from different methods, but always has the same
results. Cache it after the 1st run so subsequent runs are no-ops.

Before:

    HUB_VERBOSE=1 git browse
    $ git config alias.browse
    $ git rev-parse -q --git-dir
    $ git rev-parse -q --git-dir
    $ git remote -v
    $ git rev-parse -q --git-dir
    $ git config hub.host
    $ git rev-parse -q --git-dir
    $ git config push.default
    $ git rev-parse -q --git-path refs/remotes/origin/no-angular
    $ git rev-parse -q --git-dir
    $ git rev-parse -q --git-dir
    $ git rev-parse -q --git-dir

After (6 fewer invocations):

    HUB_VERBOSE=1 git browse
    $ git config alias.browse
    $ git rev-parse -q --git-dir
    $ git remote -v
    $ git config hub.host
    $ git config push.default
    $ git rev-parse -q --git-path refs/remotes/origin/no-angular

[ci skip]

Have `HUB_VERBOSE` also log externally run commands

Properly support `-C` git global flag

From git documentation:

    -C <path> : Run as if git was started in <path> instead of the
                current working directory. When multiple -C options are
                given, each subsequent non-absolute -C <path> is
                interpreted relative to the preceding -C <path>.

We correctly forwarded the `-C <path>` values as git global flags to git
invocations such as `git rev-parse --git-dir`, but that command is
designed to return the result as a relative path to the value computed
from `-C`. This adds an extra step to transform this relative path to an
absolute one by applying values of `-C` as the base directory.

Fixes #1015, broken after #861

[ci skip]

Add installation instructions for zsh tab-completion

Also removed instructions and references to set up git auto-completion.

Fixes #929

Fix `compare` crash when used on a non-GitHub repo

Enhance `hub ci-status` to consume and display combined statuses

CI statuses from GitHub API might not have "target_url" property.

Automatically publish a GitHub release from Travis CI

Fixes #841

Improve crash report with command and version

This adds hub version to crash report. It includes refactoring of hub version to the `version` package for reusability in `commands` package and `github` package.