Git: Factor out pattern for invalid branch names

Re-use it in the follow-up commit for remote names also.

Change-Id: Ib88792a1abdc2fa8ac23c99aa7d4861728e36eec
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/git/gitplugin.h

@@ -88,6 +88,7 @@ public:
     Gerrit::Internal::GerritPlugin *gerritPlugin() const;
     bool isCommitEditorOpen() const;
     static QString msgRepositoryLabel(const QString &repository);
+    static QString invalidBranchAndRemoteNamePattern();
     void startCommit();
     void updateBranches(const QString &repository);