git fetch composer" exceeded the timeout of 300 seconds
Composer itself impose a limit on how long it would allow for the remote git operation. Composer documentation confirms that the environment variable COMPOSER_PROCESS_TIMEOUT governs this. The variable is set to a default value of 300 (seconds) which is apparently not enough for a large clone operation using a slow internet connection.
We can resolve this problem by raising composer's process timeout value.
Then re-run the composer commands.