This is not as efficient as using ByteStrings throughout, but converting
the String to ByteString is actually significantly faster than the old
parser.
benchmarking parse/old
time 9.657 μs (9.600 μs .. 9.732 μs)
1.000 R² (0.999 R² .. 1.000 R²)
mean 9.703 μs (9.645 μs .. 9.785 μs)
std dev 231.6 ns (161.5 ns .. 323.7 ns)
variance introduced by outliers: 25% (moderately inflated)
benchmarking parse/new
time 834.6 ns (797.1 ns .. 886.9 ns)
0.987 R² (0.976 R² .. 0.999 R²)
mean 816.4 ns (802.7 ns .. 845.1 ns)
std dev 62.39 ns (37.66 ns .. 108.4 ns)
variance introduced by outliers: 82% (severely inflated)
There is a small behavior change from the old parsePOSIXTime,
which accepted any amount of trailing whitespace after the timestamp.
That behavior was not documented, and it doesn't seem anything relied on it.
Luckily, this did not affect any git-annex log files, since they all
include the trailing 's' for backwards compatability reasons.
But, if I later want to drop that, this is the first commit where
git-annex can be trusted to parse that right.
The misparse caused it to be off by up to 10 seconds.
