diff: vary default prefix depending on what are compared

With a new configuration "diff.mnemonicprefix", "git diff" shows the
differences between various combinations of preimage and postimage trees
with prefixes different from the standard "a/" and "b/".  Hopefully this
will make the distinction stand out for some people.

    "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).

Because these mnemonics now have meanings, they are swapped when reverse
diff is in effect and this feature is enabled.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 81f981509a..74af36de5c 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -581,6 +581,22 @@ diff.external::
 	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'.