meson: fix building mergetool docs

Building the documentation with meson when the build directory is
not an immediate subdirectory of the source directory prints the
following error

[2/1349] Generating Documentation/mer... command (wrapped by meson to set env)
../../Documentation/generate-mergetool-list.sh: line 15: ../git-mergetool--lib.sh: No such file or directory

The build does not fail because the failure is upstream of a pipe. Fix
the error by passing the correct source directory when meson runs
"generate-mergetool-list.sh". As that script sets $MERGE_TOOLS_DIR
we do not need to set it in the environment when running the script.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

Documentation/meson.build

@@ -352,13 +352,10 @@ foreach mode : [ 'diff', 'merge' ]
     command: [
       shell,
       '@INPUT@',
-      '..',
+      meson.project_source_root(),
       mode,
       '@OUTPUT@'
     ],
-    env: [
-      'MERGE_TOOLS_DIR=' + meson.project_source_root() / 'mergetools',
-    ],
     input: 'generate-mergetool-list.sh',
     output: 'mergetools-' + mode + '.adoc',
   )