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>

contrib/coccinelle/meson.build

@@ -1,4 +1,9 @@
-spatch = find_program('spatch', required: get_option('coccinelle'))
+coccinelle_opt = get_option('coccinelle').require(
+  fs.exists(meson.project_source_root() / '.git'),
+  error_message: 'coccinelle can only be run from a git checkout',
+)
+
+spatch = find_program('spatch', required: coccinelle_opt)
 if not spatch.found()
   subdir_done()
 endif