git-gui: fix use of GIT_CEILING_DIRECTORIES

The GIT-VERSION-GEN script sets up GIT_CEILING_DIRECTORIES so that we
won't accidentally parse version information from an unrelated parent
repository. The ceiling is derived from the source directory by simply
appendign "/.." to it, which mean that we'll only consider the current
directory for repository discovery.

This works alright in the case where git-gui is built as a standalone
project, but it breaks when git-gui is embedded into a _related_ parent
project. This is for example how git-gui is distributed via Git.

Interestingly enough, the version information is still derived properly
when building git-gui via Git's Makefile. In that case we eventually end
up specifying the ceiling directory as "./.." as we use relative paths
there, and that seems to not restrict the repository discovery.

But when building via Meson we specify the source directory as an
absolute path, and if so the repository discovery _is_ stopped. The
consequence is that we won't be able to derive the version in that case.

Fix the issue by adding a new optional parameter to GIT-VERSION-GEN that
allows the caller to override the parent project directory and wire up
new build options for Meson and Make that allows users to specify it.

Note that by default we won't set the parent project directory. This
isn't required for Meson anyway as we already use absolute paths there,
but for our Makefile it means that we still end up with "./.." as
ceiling directory, which is ineffective. But using e.g. pwd(1) as the
default value would break downstream's version generation, unless we
updated git-gui and the Makefile at the same point in time.

Signed-off-by: Patrick Steinhardt <ps@pks.im>

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