gitweb: Better processing format string in custom links in navbar

gitweb: Better processing format string in custom links in navbar

Make processing format string in custom links in action bar ('actions'
feature) more robust.  Now there would be no problems if one of
expanded values (for example project name, of project filename)
contains '%'; additionally format string supports '%' escaping by
doubling, i.e. '%%' expands to '%'.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Shawn O. Pearce <redacted>