Add user friendly output from jgit commit command

Instead of printing the stack trace from the JGitInternalException
thrown from CommitMessage.call(), print just the exception
message, using the Die exception.

Change-Id: I9ec3eb02eb001813fa15ac6f90a606223dcdafdc

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Commit.java

@@ -94,7 +94,12 @@ class Commit extends TextBuiltin {
 		commitCmd.setAmend(amend);
 		commitCmd.setAll(all);
 		Ref head = db.getRef(Constants.HEAD);
-		RevCommit commit = commitCmd.call();
+		RevCommit commit;
+		try {
+			commit = commitCmd.call();
+		} catch (JGitInternalException e) {
+			throw die(e.getMessage());
+		}
 
 		String branchName;
 		if (!head.isSymbolic())