Don't try to load magit-libgit when magit-inhibit-libgit is set

[ulm]

The context of this patch is that the app-emacs/magit package in Gentoo Linux no longer requires app-emacs/libegit2 as a dependency; see downstream bug: https://bugs.gentoo.org/910381

However, without libegit2, magit-libgit.el can neither be byte-compiled nor loaded (because it requires libgit). Therefore the Gentoo package in its default configuration no longer installs magit-libgit.

Unfortunately, this leads to a warning message when the libegit2 module is present (for unrelated reasons) on the user's system:

Error while loading ‘magit-libgit’: (file-missing "Cannot open load file" "No such file or directory" "magit-libgit")
That is not fatal.  The ‘libegit2’ module just won’t be used.

This patch suppresses that warning message by testing magit-inhibit-libgit before trying to load magit-libgit.

[tarsius]

Thanks!