In the YAML block of the PDF output, include the source document's
revision identifier for all the version control systems we use: SVN,
git-svn, and stock Git.  If uncommitted local changes are present,
indicate that too.  Relatedly, rename the commit identifier field name
to "document_rev_id", since "ots_svn" was SVN-specific.

Separately from all the above, also fix an information leak.

Explanation by example:
=======================

Before this commit, our YAML block couldn't handle regular Git trees
(such as a clone of https://code.librehq.com/ots/dosp-research), as
opposed to git-svn checkouts, and the block didn't indicate the
presence of uncommitted local changes (if any) either.  Also, it would
includ the full path to the local input file, even though that path is
specific to the user's computer and by default should be kept private.

Here's an example showing the difference between the old output and
the new output.

In a stock Git working tree with uncommitted local changes, the old
output would look like this:

  %%% BEGIN OTS YAML BLOCK %%%
  % client: ''
  % date: 15 Dec 2023
  % doctools_git_branch: main
  % doctools_git_id: commit 11ffa0c9bd9e69a3735744aafb7d4cb73fbd66e9
  % draft: false
  % input_filename: /home/jrandom/super-secret-clients/moly-cartel/smuggling/molybdenum-report.ltx
  % ots_svn: r
  % title: 'Recent Developments In The Molybdenum Market'
  % todos: []
  %%% END OTS YAML BLOCK %%%

Note the 'ots_svn' field (i.e., the field name implies SVN, even
though this is actually a Git working tree), and see how the field's
value is an empty SVN-style revision number -- just the "r" prefix.
Note also the information-leaking value of the 'input_filename' field.

Now here's the new output for the same tree:

  %%% BEGIN OTS YAML BLOCK %%%
  % client: ''
  % date: 15 Dec 2023
  % doctools_git_branch: main
  % doctools_git_id: 11ffa0c9bd9e69a3735744aafb7d4cb73fbd66e9
  % document_rev_id: 557a94fca2342957793f19c4398c89356dea98fa + uncommitted local changes
  % draft: false
  % input_filename: molybdenum-report.ltx
  % title: 'Recent Developments In The Molybdenum Market'
  % todos: []
  %%% END OTS YAML BLOCK %%%

If the tree is pristine (has no local changes), then the new output
looks like this:

  %%% BEGIN OTS YAML BLOCK %%%
  % client: ''
  % date: 15 Dec 2023
  % doctools_git_branch: main
  % doctools_git_id: 11ffa0c9bd9e69a3735744aafb7d4cb73fbd66e9
  % document_rev_id: 557a49cf2a43927597ff914c93c898356dea98fa
  % draft: false
  % input_filename: molybdenum-report.ltx
  % title: 'Recent Developments In The Molybdenum Market'
  % todos: []
  %%% END OTS YAML BLOCK %%%

You're welcome.

The venv module is part of standard python, so let's use it instead of
virtualenv, which requires its own installation step.

Make is not a great taskrunner.  There are some tasks that aren't
really about building and we don't need make for it.  So here's a simple
task runner where we can put those things.

To start, we have a 'watch' task, which just watches the doc for changes
and rebuilds as needed.  We also have a task that checks the links in a
pdf for 200 responses and complains if it doesn't get one.

Also, add a Makefile target to establish bin/dosh in new doc dirs.  That
target isn't automatically hit.  The user has to do it manually.  Maybe
we'll tie it in so it just gets added (like venv), but for now it's not
a crucial part of the workflow, so we don't do it.

The bin/dosh we stick in dirs is not just a copy and not just a symlink.
It's a wrapper around our dosh.  This lets the user inherit our targets
but also add doc-specific changes and new tasks.

This is identical to ${OTS_DIR}/forms/latex/report.ltx (as of r22457),
except that here we drop the "MACROS" section from the end -- all it
contained was the "contact()" macro, which gave James's and Karl's
contact information, which is not something we want to ship here.

This commit may or may not cause a problem for OTS folks building
reports, depending on whether this report.ltx shadows the one in
${OTS_DIR}/forms/latex/report.ltx.  If it does, we'll fix that when we
find out about it.  Right now, Karl and Seth Schoen just needed to get
ots-doctools working for Seth on his machine, and the obstacle in
front of them was that report.ltx was alleged to be missing:

  jinja2.exceptions.TemplateNotFound: report.ltx

(In the OTS SVN tree, r21800 adjusts for this change.)

The original motivation for this change was to avoid confusion with
\otsparenref and some other related commands that I'm about to add.
But then when I looked more closely, I realized that the documentation
for \otsref (now \otsoref) was quite out-of-date too, so I updated it.

Suddenly it's breaking doc builds.  I don't know why.

Before this change, refs would be stripped from documents by default
(even documents, such as invoices, that don't normally have any
explicit YAML head).  After this change, refs are not stripped by
default, but if you put "remove-refs: TRUE" in the YAML head then they
will be, and you can optionally customize the regex with "ref-regex".

Note also that it's "ref-regex" now instead of "ref_regex".  This is
because some editors (cough Emacs cough) render unescaped underscores
in a jarring and distracting way in LaTeX documents, including even
underscores in YAML frontmatter.  A better fix would be to teach Emacs
how to recognize when it's in a non-LaTeX section of a mostly-LaTeX
document, but that's a bit more yak than I have time to shave today.

Before this fix, building a document that has zero instances of
"\VAR{todo('foo')}" in it would still result in a "No todos" message
at the top (this has probably been true since commit 5a765ff9).

This is how our infrastructure improves, one yak-shave at a time.

Make "hour" and "hours" align on the right, which visually implies a
vertical line between the hours column and the charges column.  This
in turn makes right-aligning the charges column even more attractive
than it already was, so do that too (though it was probably a good
idea before and we just hadn't gotten around to doing it until now).

Persuading LaTeX (or rather, TeX) to vary its output based on singular
vs plural turned out to be a major research project.  But now we've
finally done it, so we can just sit back and let the profits roll in.