Add target repo and branch to subject of RFR email

In order to quickly tell where a PR is targeted, it would be useful if the Subject line of a PR indicated the name of the repo (e.g., jdk, jdk14, jfx, panama) in the subject line. Further, for those repos using branches, it would be helpful to include the branch name in the subject line, if it is other than the default branch (master).

This is especially helpful during rampdown of a release, where you want to quickly know whether a PR is for the mainline or the stabilizing release.