Util.pm: add git_bool function
Most helpful for interpreting a "boolean" value according to
Git semantics when dealing with read_config_file/read_config_file_hash
output.
Translates a value according to Git boolean semantics into either 0 or 1
or returns undef if it's not a boolean according to Git.
The "undef" value can be treated as true by passing an optional second
parameter that's true. This can be helpful when dealing with raw
read_config_file output where valueless booleans present with an undef value.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>