CSV is not a VCS, but CVS is

This makes build_tools/make_vcs_completions.fish not fail because 'csv' is an
unknown command, and not write a nonsense file share/completions/csv.fish.
Escape descriptions for VCS subcommands

Previously the generated darcs completions would crash as a result of unescaped
single quotes in the descriptions of subcommands.
No double quotes in long VCS options

Previously the hg completions would crash because unrelated text got parsed as
long options in the help for subcommands like 'branch'.
No lowercase in hg/darcs metavars

This fixes many generated completions for hg and darcs where previously some
descriptions had the first word cut off and the argument was considered
exclusive (`-x`).
Don't complete for non-command hg help topics

`hg help` lists some help tocis that aren't actual commands, but the script for
generating VCS completions was interpreting these as commands and producing
nonsens completions for them.
Recognize more hg metavars

This improves the accuracy and helpfulness of many of the generated completions
for hg.
Update generated darcs completions

The previous completions were outdated and crashed on load because of unescaped
quote characters in descriptions.
Update generated hg completions

This fixes a *lot* of problems where many arguments were incorrectly said to be
exclusive; where descriptions got the first word chopped off; where various
metavars weren't correctly recognized and where purely documentative help pages
got incorrectly interpreted as actual hg commands.

Some new completions also got added as I ran this against the latest release
series, 2.6.
Update generated CVS completions

This is just the same script run against the latest CVS version; I didn't
change anything in how the options are parsed.

A bunch of completions are removed in this commit, because apparently those
commands and options are not available in the latest CVS version.  I don't know
CVS so I don't know if this matters, but seeing as the latest version is five
years old already I suspect people aren't using anything much older than that.
Then again they're using CVS so who knows.