jobd.pl: avoid spawning git to read config info
The jobd.pl process needs to repeatedly read config information
(lastrefresh, lastgc) to determine whether or not to run jobs.
Instead of spawning a new "git config" process every time a
config item needs to be read, switch to the read_config_file_hash
function and cache the result as long as we're reading from the
same file with the same size and last modification timestamp.
This tremendously speeds up jobd.pl's ability to check and skip
projects which do not need to run either update or gc.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>