Ported from my bash version over at
https://github.com/mislav/dotfiles/blob/370154a5adc482afd89606c126bcd18df2560013/bin/git-sync

Fixes #1249

With the new approach, create a temporary remote, fetch it, and delete
it immediately. Its git objects will still remain to be available for
cherry-picking.

E.g. `/OWNER/REPO/pull/123/commits/SHA`

Fixes #1249

Fixes weird failure that happens only on Windows:

    cannot use syscall.Stdin (type syscall.Handle) as type int in
    argument to terminal.GetState

I would guess `syscall.Stdin` is of different type on Windows and that
this might be a Go bug.

After the last howeyc/gopass update, hitting Enter after password input
makes the next printed line appear indented to the position of the
password prompt. I'm not sure what causes this, but as a workaround,
skip the howeyc/gopass dependency altogether and interface directly with
crypto/ssh/terminal.

Fixes #1262

See https://groups.google.com/forum/#!topic/golang-nuts/kTVAbtee9UA

Enable copy to clipboard functionality for commands that print URLs

Also add `--browse` functionality to `create` and `release` for parity
with `issue` and `pull-request`.

This is to free up `-c` for `--copy`.

The old `args.After("echo", ...)` approach won't cut it on Windows since
there is no `echo` executable. Thus, nothing was printed on Windows.

Instead, expand Args type with extra functionality that allows Go
callbacks after all commands in the chain have been run.

Also, the new `args.NoForward()` is now preferred to `os.Exit()` since
it's more descriptive and allows `defer` in the calling function.

If shelling out to nonexistent command, such as `echo` on Windows, print
the error on stderr instead of failing silently. This is also helpful
when hub tries to forward a command to `git`, but git is not installed.

The Windows installer appends the location of `hub.exe` to user's PATH,
but that would break if the OS username has spaces.

Fixes #1254

Fixes #1256

The GitHub API doesn't seem to like receiving `"assignees": null`.

Fixes #1240

Fixes #1243

When `api.PostJSON()` returns an error because of invalid JSON or a
network error, `res` variable will be nil. Avoid reading from nil `res`.

Fixes #1253, fixes #1251, fixes #1248, fixes #1244

Don't try to default `target_commitish` to the currently checked out
branch. Instead, keep the value blank unless explicitly provided. It
will default to the main branch on the server.

Fixes #446, fixes #937