As pointed out in bug 784209, the --module-dir should be the 'html'
directory, which wasn't a problem with 1.25, but git master of gtk-doc
fails to find html files when it was set to the current directory.
Similar to NEWS-file test, add also SKIP_COMMIT_TEST=1 to be able to skip
any such issue. This helps to notice uncommitted changes before the tarball
is created.
It verifies that the distribution tarball is created with all local
changes, which requires them to be committed. If there are uncommitted
changes, then it prints a warning on the console.