build: guard git version detection against missing .git directory

When building from a release tarball, there is no .git directory
and git rev-parse would either fail or pick up a parent repository's
HEAD. Guard the git version detection with fs.is_dir('.git') so
that tarball builds produce a clean version string.

Requires importing the fs module.

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>
(cherry picked from commit e5db9506b4)

meson.build

@@ -18,6 +18,8 @@ add_project_arguments(
   language: 'c',
 )
 
+fs = import('fs')
+
 if get_option('buildtype').startswith('debug')
   add_project_arguments('-DDEBUG', language : 'c')
 endif
@@ -43,6 +45,7 @@ math           = cc.find_library('m')
 have_xwayland = wlroots.get_variable(pkgconfig: 'have_xwayland', internal: 'have_xwayland') == 'true'
 
 version = '@0@'.format(meson.project_version())
+if fs.is_dir('.git')
   git = find_program('git', native: true, required: false)
   if git.found()
     git_commit = run_command([git, 'rev-parse', '--short', 'HEAD'], check: false)
@@ -55,6 +58,7 @@ if git.found()
       )
     endif
   endif
+endif
 
 conf_data = configuration_data()
 conf_data.set10('CAGE_HAS_XWAYLAND', have_xwayland)