meson: tolerate errors from git ls-files --deduplicate

meson: tolerate errors from git ls-files --deduplicate

When using the Meson build system with versions of Git before 2.31,
that does not yet know the `git ls-files --deduplicate` option, one
can observe the following error:

    ../meson.build:697:19: ERROR: Command `/usr/bin/git -C /home/martin/code/git ls-files --deduplicate '*.h' ':!contrib' ':!compat/inet_ntop.c' ':!compat/inet_pton.c' ':!compat/nedmalloc' ':!compat/obstack.*' ':!compat/poll' ':!compat/regex' ':!sha1collisiondetection' ':!sha1dc' ':!t/unit-tests/clar' ':!t/t[0-9][0-9][0-9][0-9]*' ':!xdiff'` failed with status 129.

The failing command is used to find all header files in our code
base, which is required for static analysis.

Static analysis is an entirely optional feature that distributors
typically don't care about, and we already know to skip running the
command when we are not in a Git repository. But we do not handle
the above failure gracefully, even though we could.

Fix this by passing `check: false` to `run_command`, which makes it
tolerate failures. Then check `returncode()` manually to decide
whether to inspect the output.

Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Junio C Hamano <gitster@pobox.com>