mesa: Fix generation of git_sha1.h.tmp for gitlinks

mesa: Fix generation of git_sha1.h.tmp for gitlinks

Don't assume that $(top_srcdir)/.git is a directory. It may be a
gitlink file [1] if $(top_srcdir) is a submodule checkout or a linked
worktree [2].

[1] A "gitlink" is a text file that specifies the real location of
    the gitdir.
[2] Linked worktrees are a new feature in Git 2.5.

Cc: "10.6, 10.5" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>