The patch utility is used to merge a diff file into your own source tree. This tool is only needed, if you want to apply a patch (diff file) from someone else (probably from the developer mailing list) to try out in your own private source tree.
It most cases you may not need the patch tool installed. Git should handle patches for you.
Patch is available for most UNIX and UNIX-like platforms. If GNU patch isn’t already installed or available as a package for your platform, you can get it at https://www.gnu.org/software/patch/patch.html.
After correct installation, typing at the bash command line prompt:
$ patch --version
should result in something like:
patch 2.5.8 Copyright (C) 1988 Larry Wall Copyright (C) 2002 Free Software Foundation, Inc. This program comes with NO WARRANTY, to the extent permitted by law. You may redistribute copies of this program under the terms of the GNU General Public License. For more information about these matters, see the file named COPYING. written by Larry Wall and Paul Eggert
However, the version string may vary.
The Windows native Git tools provide patch. A native Windows patch package can be obtained from http://gnuwin32.sourceforge.net/. The installation should be straightforward.