contrib: add "hgperf" command to measure performance of commands easily
Newly added "hgperf" command repeats "dispatch.runcommand()" for
specified Mercurial command and measure performance of it in the same
manner of "contrib/perf.py" extension.
Users (mainly developers) can examine performance of the target
command easily, without adding new entry for it to "contrib/perf.py"
extension.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null