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>