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.

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

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>

Fixes #463 and #948

Code of Conduct

Avoid depending on a hardcoded list of git global flags

Avoids mutating `globalFlags` until a match has actually been found.

Update EDITMSG copy

Before, the argument parser would translate `--help` and `--version` to
their equivalents without the dashes. Now it's not the parser's
responsibility to do that anymore. Instead, wire them up as subcommand
aliases in the runner.

This relieves `slurpGlobalFlags` of having too many duties

We used to recognize git global flags based on a hardcoded list.
Whenever git added more global flags, we didn't support them in hub
until we updated the list.

Now, treat all flags coming before the subcommand as global flags. These
are the rules:

1. Initially, every argument except `--help` and `--version` that starts
   with a `-` is considered to be a global flag.

2. Additionally, `-c` and `-C` flags have a value.

3. When an argument appears that doesn't match the above rules, assume
   that it's the git subcommand and stop global args parsing.

Fix spurious test failure when GIT_EDITOR is set

`git var` checks the `$GIT_EDITOR` environment variable before the `core.editor` config knob. `TestGitDir` intends to verify that the latter is respected, so sanitize the environment before running the test and restore it upon completion.

Support coloring console on Windows

Introduce github.com/mattn/go-colorable to solve the console color
issues on Windows

bitbucket.org/kardinaos/osxt was removed and github.com/inconshreveable/go-update has changed to use github.com/kardianos/osext. Not bumping go-update will prevent upgrade of other packages with `godep save -r ./…`