[Integrated] RFR: git-pr: do not assume a master branch

[Erik Helin]

Changeset: 4637c1b5
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-02-10 09:42:02 +0000
URL:       https://git.openjdk.java.net/skara/commit/4637c1b5

git-pr: do not assume a master branch

Reviewed-by: rwestberg

! cli/src/main/java/org/openjdk/skara/cli/GitPr.java
! vcs/src/main/java/org/openjdk/skara/vcs/ReadOnlyRepository.java
! vcs/src/main/java/org/openjdk/skara/vcs/git/GitRepository.java
! vcs/src/main/java/org/openjdk/skara/vcs/hg/HgRepository.java