manifest_changed.sh intermittently returns 1 but reports no actual changes

[jdm]

bash ./etc/ci/manifest_changed.sh
 in dir /Users/servo/buildbot/slave/mac-dev-unit/build (timeout 1200 secs)
 watching logfiles {}
 argv: ['bash', './etc/ci/manifest_changed.sh']
 environment:
  CARGO_HOME=/Users/servo/.cargo
  CCACHE=/usr/local/bin/ccache
  HOME=/Users/servo
  LOGNAME=servo
  OPENSSL_INCLUDE_DIR=/usr/local/opt/openssl/include
  OPENSSL_LIB_DIR=/usr/local/opt/openssl/lib
  PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
  PWD=/Users/servo/buildbot/slave/mac-dev-unit/build
  RUST_BACKTRACE=1
  SERVO_CACHE_DIR=/Users/servo/.servo
  SHELL=/bin/bash
  TMPDIR=/var/folders/ql/81t7wn396z92cblm26x24qp40000gp/T/
  USER=servo
  XPC_FLAGS=0x0
  XPC_SERVICE_NAME=buildbot-slave
  __CF_USER_TEXT_ENCODING=0x1F6:0x0:0x0
 using PTY: False
program finished with exit code 1
elapsedTime=78.842138