meson: fix generation of merge tools

Our buildsystems generate a list of diff and merge tools that ultimately
end up in our documentation. And while Meson does wire up the logic, it
tries to use the TOOL_MODE environment variable to set up the mode. This
is wrong though: the mode is set via an argument that we have fixed to
'diff' mode by accident.

Fix this such that merge tools are properly generated.

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

Documentation/meson.build

@@ -318,12 +318,11 @@ foreach mode : [ 'diff', 'merge' ]
       shell,
       '@INPUT@',
       '..',
-      'diff',
+      mode,
       '@OUTPUT@'
     ],
     env: [
       'MERGE_TOOLS_DIR=' + meson.project_source_root() / 'mergetools',
-      'TOOL_MODE=' + mode,
     ],
     input: 'generate-mergetool-list.sh',
     output: 'mergetools-' + mode + '.txt',