test if --local can be used by "git config", fixes #120 #73 #44 ?

test if --local can be used by "git config" if not use "git config" with...out --local, fixes #120 #73 #44 ?

@nirvdrum said in https://github.com/nirvdrum/svn2git/pull/44#issuecomment-4176673 that you'd be happy to accept a PR which tries --local first and then falls back. Here it is tried once instead of in each call to run_command("git config ...").