gitweb: support automatic readme generation
If enabled by setting $git_automatic_readme_html to an executable
that takes a .git directory as its argument, then if README.html
insertion is enabled and README.html does not exist or is zero
length, then the output of the $git_automatic_readme_html utility
will be inserted instead if it is non-empty.
It's still possible to prevent display of any readme by creating
a README.html file that consists of only XML <!-- comments -->.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>