build: Add a Meson build system

[smcv]

• man: Include source code for the man page in dist tarballs

• rust: Fix build of git-rustevtag

  It's now built as git-evtag by the Cargo build system, so we need to
  rename it as part of copying it.

• tests: Be more strict about producing TAP output

  Autotools' TAP parser ignores unknown content on stdout, but Meson is
  much more strict and considers unknown content to be an error. Redirect
  commands' stdout to stderr if they can produce arbitrary unstructured
  diagnostics on stdout.

• build: Add a Meson build system

[smcv]

@cgwalters, are you interested in adding a Meson build system, and potentially dropping the Autotools build at some point?

(If not, I can move the first three commits into a separate PR.)

[cgwalters]

Nice! Dropping autotools seems fine to me. Thanks so much for all your work on this!

[eli-schwartz]

• tests: Be more strict about producing TAP output
  Autotools' TAP parser ignores unknown content on stdout, but Meson is
  much more strict and considers unknown content to be an error. Redirect
  commands' stdout to stderr if they can produce arbitrary unstructured
  diagnostics on stdout.

Note: The TAP specification says that unknown lines are never parser errors, in order to allow for forward compatibility with new versions of TAP, but they may be harness errors, and some harnesses error, some ignore it, and some non-fatally warn about it.

The intention of unknown lines is, regardless, intended specifically for not-yet-implemented TAP keywords, and arbitrary content emitted by programs run within the test should be prefixed with # as the first character on the line, in order to be parsed as TAP diagnostics (that is, comments, which cannot affect the tests themselves but may of course be displayed by the harness for informational purposes).