git-pull: add the new create-pull-request script

git-pull: add the new create-pull-request script

The previous create-pull-request only generated a cover letter. When used to
send to the list, it did not include the patches, which made it difficult
to perform peer review. A pull request without patches is typically only sent
by a maintainer. As we are not all maintainers, we need a means to easily
submit patches for review.

As we are accustomed to making pull requests, this script retains a
git-pull-style cover letter, while sending the relevant patches as responses
to the pull. This will provide the necessary context for peer review, and still
allow people to collapse threads and see no more mail than they were previously.

This version retains the relative_to, commit_id, and contrib_branch arguments
from the original, along with their default values. It adds several more,
resulting in a highly flexible tool.

The script creates a pull directory (pull-$$ by default, configurable via the -o
option) and populates it with a git-format-patch generated patch series and
cover letter. The cover letter is modified to include the git and http pull URLs
and branch name, as well as a basic signature from the author pulled from git's
user.name and user.email config. git-format-patch provides the shortlog and
diffstat of the series.

Breaking a bit from the original, this script maintains the [PATCH] subject
prefix in the cover letter (as opposed to [GIT PULL]. This is better suited to
the majority of developers (who are not maintainers). This prefix is
configurable with the -p option, allowing you to create an [RFC PATCH]
prefix, for example.

By default, the generated cover letter with contain "*** SUBJECT HERE ***" and
"*** BLURB HERE ***" tokens which you should replace with something
appropriate prior to sending the messages.

When developing multiple versions of a patch series, it can save time to
maintain a message.txt file, rather than having to retype the message body of
the cover letter every time. The -m option allows you to specify a message file
and replace the "*** BLURB HERE ***" token of the cover letter with the contents
of the message file.

Finally, the -s option will replace the "*** SUBJECT HERE ***" token in the cover
letter with the specified subject.

The generated patches are suitable for sending via sendmail.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Nitin A Kamble <nitin.a.kamble@intel.com>
CC: Richard Purdie <rpurdie@linux.intel.com>
CC: Saul Wold <saul.wold@intel.com>
CC: Bruce Ashfield <bruce.ashfield@windriver.com>