Fixes #1724.

This patch omits the "Signed-off-by:" line coming from pull requests created
out of single commit messages. This way the user does not need to manually
remove their email address from the PR description.

Not introducing a flag for this as it applies to only one case (single-commit
diff between heads, as well as no -m/-f/-i specified).

Meta: sending this PR with the patched build, my Signed-off-by line is removed
automatically from the PR message.
Signed-off-by: NAhmet Alp Balkan <ahmetb@google.com>

This shows commented-out text such as "Creating an issue for github/hub"
not only when `-m` or `-F` weren't passed, but also when they were
passed and `--edit` was used.

The commit body comes first, then template text. This is for
compatibility with GitHub web interface.

Fixes #1657

Previously, passing `--edit` in combination with `-m` would skip opening
the editor. For compatibility with `git commit`, `--edit` is now always
respected.

Before, the code outside of `editor.go` was responsible for prefixing
lines of the commented section with the comment char. The code to do
this was scattered all over the codebase and was difficult to refactor
re: adding support for `commentchar=auto`.

Now, Editor has a `AddCommentedSection()` method that allows for
declarative adding of a commented section. Furthermore, the caller
doesn't have to query git to determine the comment char.

`git rev-parse --show-toplevel` returns an empty string with success
status when inside a bare git repo.

This avoids the go crash and also tweaks `hub issue` and `hub
pull-request` to work even if current working directory name couldn't be
obtained because it's only used for looking up issue/PR templates, which
isn't critical functionality.

Fixes https://github.com/github/hub/issues/1331

This ensures that a branch pushed using "hub pull-request" has its
upstream branch set to the target of the push.

Always start template lookup from the toplevel directory of the repo.

Fixes #1242

Since `--push` has just created a new branch, there could be a slight
delay before GitHub registers it, and there's a possibility that a 422
would be returned for the "head" field. If that happens, retry the POST
a couple of times until it either succeeds or times out.

This removes an extra step from the process of publishing a pull
request. It also gives the user a chance to back out of the entire
process by saving an empty pull request message.

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

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.

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

Fixes #1240

This allows accumulating values across repeated arguments.

When the repo that hosts the `head` for the PR is moved to another
owner, the `owner:branch` specification will be wrong and the GitHub API
will return HTTP 422.

This resolves the latest owner/name pair for head repo so that the value
is never stale when submitting new PR to be created.

Fixes #467

Implements the GeneratePRTemplate in pull_request.go instead of the
custom logic that previously existed in the file.  Addresses:

* Title being defaulted as the first line of the template
* Commit messages not defaulting as the title only properly

The description for --head said it was the head branch instead of head branch.

Fixes #1057