As mentioned in the diff man page, a/ and b/ represent the prefix to differentiate source and destination.
Actually, you have the options:
--no-prefix
Do not show any source or destination prefix.
--src-prefix=<prefix>
Show the given source prefix instead of “a/”.
--dst-prefix=<prefix>
Show the given destination prefix instead of “b/”
Gringo Suave adds in the comments:
Went with:
git diff --src-prefix="SRC " --dst-prefix="DST "to make double-click select and paste work.