Add man page for git pull-request

[rishitank]

It would be great to have a list of commands in a help page when using git pull-request --help to know the full extent of the functionality of this package, thanks.

[rishitank]

It appears that git pull-request --help is not aliased with git pull-request -h

[jd]

It's just that git pull-request --help calls git which tries to run the manpage.