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.

Now `ci-status` consumes the new combined statuses API endpoint designed
for when multiple contexts report statuses independently.

The verbose output is now colorized and is in format where each line is:

    ✔︎<Tab>[CONTEXT]<Tab>[URL]

Update `script/bootstrap` to recommended Go and Ruby version

[ci skip]

Closes #970

We can't use `--git-path` on git versions older than 2.5. Those versions
will simply echo `--git-path` as the first line of the output. Instead
of stat'ing that as a file, skip this case and proceed immediately to
fallback for other git versions.

Git 1.5.0 introduces support for worktrees. A worktree has a
.git file, rather than a folder. (The file contents points to to a
.git folder buried underneath the .git folder of the main repository.)

A new option has been added to resolve paths within the git dir,
`git rev-parse --git-path foo/bar`.

The commit uses this new facility, but falls back to the old approach
for backwards compability.

Fixes #969

Windows batch script heavily lifted from git-lfs,
authors @technoweenie @joshvera
https://github.com/github/git-lfs/blob/0de6370b50962794a8195749b9187d74499a6fc9/script/install.bat.example

We don't need `gimme` anymore to cross-compile binaries.

Otherwise, it was re-read from file every time CurrentConfig() was accessed.

Allow --ff with merge

Closes #973

Signed-off-by: NDavid Calavera <david.calavera@gmail.com>

Add the `-M/--milestone` and `-l/--labels` flags to `hub pull-request` to
set the milestone or tags when a pull request is created.
Signed-off-by: NDavid Calavera <david.calavera@gmail.com>