Allow cgitrc variables to be set in Git config

Although repo-specific variables can already be set in the Git
configuration file, global CGit variables cannot.  Give a way to
specify variables for git-instacgit's cgitrc to be specified in the Git
configuration file by prefixing them with "instacgit.config.".

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'apply-git-config' into pu

* branch apply-git-config:
  Allow cgitrc variables to be set in Git config

Allow global cgitrc variables to be set in Git config for
git-instacgit.