histedit: use better meta-variable names than VALUE in help text
Before this change hg help histedit would use the default variable label:
--commands VALUE
...
-r --rev VALUE [+]
With this change the text will be in the usual help text style and a bit more
explanatory:
--commands FILE
...
-r --rev REV [+]
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null