PERF: perf improvments in indexing with object dtypes (GH5968)
BUG: Bug in NaT comparison if a mixed datetime/np.datetime64 with NaT were passed (5968)
CLN: cleanup nat comparisons in tslib.pyx
Merge pull request #5973 from jreback/format_perf

PERF: perf improvments in indexing with object dtypes (GH5968)