meson: disable coccinelle configuration when building from a tarball

Wiring up coccinelle in the build, depends on running git commands to
get the list of files to operate on. Reasonable, for a feature mainly
used by people developing on git. If building git itself from a tarball
distribution of git's own source code, one likely does not need to run
coccinelle.

But running those git commands failed, and caused the build to error
out, if `spatch` was installed -- because the build assumed that its
presence indicated a desire to use it on this source tree. Instead, we
can expand the conditional to check for both `spatch` and the `.git`
file or directory.

Meson's `opt.require()` method allows us to add a prerequisite for the
feature option. If the prerequisite fails, then the option either:

- converts autodetection to disabled

- emits an informative error if the feature was set to enabled:
  ```
  ERROR: Feature coccinelle cannot be enabled: coccinelle can only be run from a git checkout
  ```

Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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