Improve git commit hash lookup (#5225)

- Also get the branch name.
- Use rev-parse instead of describe to get a clean hash.
- Return the git hash and branch name in server_info for admin
  connections.
- Include git hash and branch name on separate lines in --version.

include/xrpl/protocol/jss.h

@@ -167,6 +167,7 @@ JSS(blobs_v2);                // out: ValidatorList
 JSS(books);                   // in: Subscribe, Unsubscribe
 JSS(both);                    // in: Subscribe, Unsubscribe
 JSS(both_sides);              // in: Subscribe, Unsubscribe
+JSS(branch);                  // out: server_info
 JSS(broadcast);               // out: SubmitTransaction
 JSS(bridge_account);          // in: LedgerEntry
 JSS(build_path);              // in: TransactionSign
@@ -290,6 +291,7 @@ JSS(frozen_balances);         // out: GatewayBalances
 JSS(full);                    // in: LedgerClearer, handlers/Ledger
 JSS(full_reply);              // out: PathFind
 JSS(fullbelow_size);          // out: GetCounts
+JSS(git);                     // out: server_info
 JSS(good);                    // out: RPCVersion
 JSS(hash);                    // out: NetworkOPs, InboundLedger,
                               //      LedgerToJson, STTx; field