Pager invocation should respect pre-existing LESS

[afranchuk]

We hard-code the LESS environment variable here, however this is not the same as git's behavior, which will only set these variables when they are currently unset.

I'm doing this because I want to add a set of highlights when doing local auditing (to call out important things to audit). If I could, I'd just override PAGER, but it looks like we only look at the configured git pager.

[mystor]

That seems like it should be easy enough to fix, if we change the behaviour to more closely match git's.