meson: fix building mergetool docs

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>