gitweb: Support legacy options used by caching engine
Try to translate between config variables used by gitweb caching
patches[1] by John 'Warthog9' Hawley (J.H.) used, among others, by
git.kernel.org, and new %cache_config options, but only if they were set
in the config file.
Note that $cache_enable is *not* translated to $caching_enabled.
Footnotes:
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[1] See for example "Gitweb caching v7" thread on git mailing list:
http://thread.gmane.org/gmane.comp.version-control.git/160147
Signed-off-by: Jakub Narebski <jnareb@gmail.com>