With the Hub command-line wrapper you can link it to git and then you can do
git pull-request
From the man page of hub:
git pull-request [-f] [TITLE|-i ISSUE|ISSUE-URL] [-b BASE] [-h HEAD]
Opens a pull request on GitHub for the project that the "origin" remote points to. The default head of the pull request is the current branch. Both base and head of the pull request can be explicitly given in one of the following formats: "branch", "owner:branch",
"owner/repo:branch". This command will abort operation if it detects that the current topic branch has local commits that are not yet pushed to its upstream branch on the remote. To skip this check, use -f.
If TITLE is omitted, a text editor will open in which title and body of the pull request can be entered in the same manner as git commit message.
If instead of normal TITLE an issue number is given with -i, the pull request will be attached to an existing GitHub issue. Alternatively, instead of title you can paste a full URL to an issue on GitHub.