meson: also allow setting GIT_VERSION via templates

GIT_VERSION is not available as a config.h variable, because it's rendered
into version.h during builds. Let's rework jinja2 rendering to also
parse version.h. No functional change, the new variable is so far unused.

I guess this will make partial rebuilds a bit slower, but it's useful
to be able to use the full version string.

shell-completion/bash/meson.build

@@ -14,7 +14,7 @@ custom_target(
         'systemctl',
         input : 'systemctl.in',
         output : 'systemctl',
-        command : [meson_render_jinja2, config_h, '@INPUT@', '@OUTPUT@'],
+        command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
         install : bashcompletiondir != 'no',
         install_dir : bashcompletiondir)