Fix loading GTFS frequencies with < 60 seconds headway #1197

Job	Run time
linux (linux-amd64-release, linux-amd64)	45s
formatting	48s
linux (linux-arm64-release, linux-arm64, qemu-aarch64-static)	48s
msvc (Debug)	5m 51s
macos (macos-x86_64, On)	7m 56s
linux (clang-tidy)	2m 21s
msvc (Release)	5m 32s
macos (macos-arm64)	7m 31s
linux (linux-sanitizer)	1m 30s
linux (linux-debug, valgrind --leak-check=full --error-exitcode=1)	2m 21s
	35m 23s

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