you want to use an external diff program only on a subset of
your files, you might want to use linkgit:gitattributes[5] instead.
+diff.mnemonicprefix::
+ If set, 'git-diff' uses a prefix pair that is different from the
+ standard "a/" and "b/" depending on what is being compared. When
+ this configuration is in effect, reverse diff output also swaps
+ the order of the prefixes:
+'git-diff';;
+ compares the (i)ndex and the (w)ork tree;
+'git-diff HEAD';;
+ compares a (c)ommit and the (w)ork tree;
+'git diff --cached';;
+ compares a (c)ommit and the (i)ndex;
+'git-diff HEAD:file1 file2';;
+ compares an (o)bject and a (w)ork tree entity;
+'git diff --no-index a b';;
+ compares two non-git things (1) and (2).
+
diff.renameLimit::
The number of files to consider when performing the copy/rename
detection; equivalent to the 'git-diff' option '-l'.