Even farther through the work for 2.1, the Changelog really needed
updating.

Also remove some trailing space.

Closes: nojhan/liquidprompt#675

Fixes https://github.com/nojhan/liquidprompt/issues/600.

Partially through the work for 2.1, it seemed like it was time to update
the Changelog.

Bump version number in Changelog.
Fix typos in Changelog.
Fix incorrect (and broken) commit reference in Changelog.
Add documentation contributors in Contributors.
Add two missed contributors in Contributors.

This is cutting v2.0.0-beta.1. There are a few fixes and small features
left for 2.0.0, but this is the bulk of it.

Fossil 1.11 changed the behavior of 'fossil info' to always succeed even
if not in a repo (just giving basic global config details). This broke
how we used it, to tell if we were in a repo.
Replacing with 'fossil status' gives the same functionality.

I should have caught this myself as I am working with fossil for the
rework, but I accidentally fixed this by using the fossil 1.7+ command
'fossil branch current'. This will work until the rework is done.

Fixes #626

This needs improvement soon, but I didn't want to completely rework this
function yet. Better to provide too much warning than too little at this
point I think.

Ideally in the future we would generate empty color vars so that
printing speed would be even better without the escape codes.

Fixes #615

Since Liquidprompt uses the DEBUG trap to calculate runtime of commands,
we run into issue when the command line is actually multiple commands.
Bash runs the DEBUG trap before each command, not each command line,
which was making our time calculations not work at all.

Using a hack inspired by this post:
https://jichu4n.com/posts/debug-trap-and-prompt_command-in-bash/
We set a flag _LP_AT_PROMPT var when we are running PROMPT_COMMAND, and
only then in the DEBUG trap do we start the timer.

Fixes #614

This one turned out to be so simple. I love fixes like that, but it
always feels bittersweet that it took so long to discover the simple
fix.

We aren't the first to run into this issue. There is a Stackoverflow
answer here: https://stackoverflow.com/a/40944702/8338372,
and another project using that solution:
https://github.com/rcaloras/bash-preexec.

The way they fixed it though is more than what is actually needed. Those
solutions assumed that the very last command run in the trap needed to
have the same $_, but that is not the case. This is pretty easy to
realize when you run `echo $_`, as it prints "_lp_runtime_before". It
doesn't print the last thing ran in that function.

So the fix is simple. Pass in the $_ as the last argument to
_lp_runtime_before in the trap definition, and it just works.

Fixes #451

"git diff" can take both refs and filenames, so if there a file named
HEAD, it would error not knowing which you wanted. Add the notation to
tell git we want the ref.

I checked and tested all the other git calls we are using, and this was
the only one that could take both filenames and refs, so therefore the
only one that needed changes.

Fixes #517

Contributors were tracked in the CHANGES file for each release, as well
as being tracked in liquidprompt itself, which not only cluttered the
file, but most of the contributors weren't listed at all.
This new file should list ALL contributors, both code and Github issue
kind. (This again took forever).
The going forward plan is to add contributors to this file, and also to
call out specific contributors with each release notes in the release
tag.

Update README to point to new CONTRIBUTORS file.
Tweak the .mailmap file to remove pointless duplicate lines.