added update functionality, can do fetch part of pull

src/main/java/cli/CommandLineInterface.java

src/main/java/command/CommandPull.java

src/main/java/command/CommandPush.java

src/main/java/command/CommandSelector.java

src/main/java/command/CommandUpdate.java

src/main/java/git/IssueRepo.java

src/main/java/git/IssueTreeBuilder.java

src/main/java/issueData/Issue.java

src/main/java/issueData/IssueRepository.java