* This goes into session.vcs as well.
* `fossil info` does not allow printing only the local-root property, so we have to do
  some parsing afterwards.

* In fact, since SVN has --no-newline, this is even simpler than on Git and Mercurial.
* This requires at least Subversion v1.9 (2015, so should be safe).

* The previous convention of !* ... *! are now true block comments,
  i.e. they are parsed faster, don't spam the goto table and allow
  embedding of exclamation marks - only "*!" terminates the comment.
* It is therefore now forbidden to have goto labels beginning with "*".
* Also support "!!" to introduce EOL comments (like C++'s //).
  This disallows empty labels, but they weren't useful anyway.
  This is the shortest way to begin a comment.
* All comment labels have been converted to true comments, to ensure
  that syntax highlighting works correctly.
  EOL comments are used for single line commented-out code, since it's
  easiest to uncomment - you don't have to jump to the line end.
  This is a pure convention / coding style.
  Other people might do it differently.
* It's of course still possible to abuse goto labels as comments
  as TECO did for ages.
* In lexing / syntax highlighting, labels and comments are highlighted differently.
* When syntax highlighting, a single "!" will first be highlighted as a label
  since it's not yet unambiguous. Once you type the second character (* or !),
  the first character is retroactively styled as a comment as well.

It's a bit easier on the eyes.

* SCI_SETILEXER(NULL) is not a reliable way to do that since
  that's the default for all views.
* This was breaking the git.tes lexer for instance and was unnecessarily
  driving teco_lexer_style() on plain-text documents.
* Since we currently do not implement the ILexer5 C++ interface
  and teco_view_t is just a pointer alias, we are abusing the view's "identifier" instead.
  This is probably sufficient, as long as there is only one lexer "in the container".
  Otherwise, there should perhaps be a single C++ class that does nothing but
  wrapping a callback into an ILexer5 object with a C ABI.

* this works by embedding the SciTECO parser and driving it always (exclusively)
  in parse-only mode.
* A new teco_state_t::style determines the Scintilla style for any character
  accepted in the given state.
* Therefore, the SciTECO lexer is always 100% exact and corresponds to the current
  SciTECO grammer - it does not have to be maintained separately.
  There are a few exceptions and tweaks, though.
* The contents of curly-brace escapes (`@^Uq{...}`) are rendered as ordinary
  code using a separate parser instance.
  This can be disabled with the lexer.sciteco.macrodef property.
  Unfortunately, SciTECO does not currently allow setting lexer properties (FIXME).
* Labels and comments are currently styled the same.
  This could change in the future once we introduce real comments.
* Lexers are usually implemented in C++, but I did not want to draw in C++.
  Especially not since we'd have to include parser.h and other SciTECO headers,
  that really do not want to keep C++-compatible.
  Instead, the lexer is implemented "in the container".
  @ES/SCI_SETILEXER/sciteco/ is internally translated to SCI_SETILEXER(NULL)
  and we get Scintilla notifications when styling the view becomes necessary.
  This is then centrally forwarded to the teco_lexer_style() which
  uses the ordinary teco_view_ssm() API for styling.
* Once the command line becomes a Scintilla view even on Curses,
  we can enabled syntax highlighting of the command line macro.

* As known from DEC TECO, but extended to convert absolute positions to line numbers as well.
  :^Q returns the current line.
* Especially useful in macros that accept line arguments,
  as it is much shorter than something like
  ^E@ES/LINEFROMPOSITION//+Q.l@ES/POSITIONFROMLINE//:^E-.
* On the other hand, the fact that ^Q checks the line range means we cannot
  easily replace lexer.checkheader with something like
  [:J 0,^Q::S...$ ]:
  Using SCI_POSITIONFROMLINE still has the advantage that it returns `Z` for out-of-bounds ranges
  which would be cumbersome to write with the current ^Q.
* Perhaps there should be a separate command for converting between absolute lines and positions
  and :^Q should be repurposed to return a failure boolean for out-of-range values?
* fnkeys.tes could be simplified.

* The colon modifier can now occur 2 times.
  Specifying `@` more than once or `:` more than twice is an error now.
* Commands do not check for excess colon modifiers - almost every command would have
  to check it. Instead, a double colon will simply behave like a single colon on most
  commands.
* All search commands inherit the anchored semantics, but it's not very useful in some combinations
  like -::S, ::N or ::FK.
  That's why the `::` variants are not documented everywhere.
* The lexer.checkheader macro could be simplified and should also be faster now,
  speeding up startup.
  Eventually this macro can be made superfluous, e.g. by using 1:FB or 0,1^Q::S.