Git internal command line parser

#=< TLDR: Where's the code for git's internal command line parser? >===#

I'm interested in the internals of git, more specifically it's command
line parser.

Does Git use a library like getopt for it's command line handling, or 
does it use a custom parser?

If possible, I would appreciate being redirected to the source code 
files that handle this.

Sincerely,
Lemuria