meson: wire up generation of distribution archive

Meson knows to generate distribution archives via `meson dist`. In
addition to generating the archive itself, this target also knows to
compile and execute tests from that archive, which helps to ensure that
the result is an adequate drop-in replacement for the versioned project.

While this already works as-is, one omission is that we don't propagate
the commit that this is built from into the resulting archive. This can
be fixed though by adding a distribution script that propagates the
version into the "version" file, which GIT-VERSION-GEN knows to read if
present.

Use GIT-VERSION-GEN to populate that file. As the script is executed in
the build directory, not in the directory where we generate the archive,
we have to use a shell to resolve the "MESON_DIST_ROOT" environment
variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -1947,6 +1947,19 @@ devenv.set('GIT_BUILD_DIR', meson.current_build_dir())
 devenv.prepend('PATH', meson.current_build_dir() / 'bin-wrappers')
 meson.add_devenv(devenv)
 
+# Generate the 'version' file in the distribution tarball. This is used via
+# `meson dist -C <builddir>` to populate the source archive with the Git
+# version that the archive is being generated from.
+meson.add_dist_script(
+  shell,
+  '-c',
+  '"$1" "$2" "$3" --format="@GIT_VERSION@" "$MESON_DIST_ROOT/version"',
+  'GIT-VERSION-GEN',
+  shell,
+  meson.current_source_dir() / 'GIT-VERSION-GEN',
+  meson.current_source_dir(),
+)
+
 summary({
   'curl': curl.found(),
   'expat': expat.found(),