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, 3 insertions, 0 deletions

diff --git a/diff-lib.c b/diff-lib.c
index e7eaff9..ae96c64 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -63,6 +63,8 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			      ? CE_MATCH_RACY_IS_DIRTY : 0);
 	char symcache[PATH_MAX];
 
+	diff_set_mnemonic_prefix(&revs->diffopt, "i/", "w/");
+
 	if (diff_unmerged_stage < 0)
 		diff_unmerged_stage = 2;
 	entries = active_nr;
@@ -469,6 +471,7 @@ int run_diff_index(struct rev_info *revs, int cached)
 	if (unpack_trees(1, &t, &opts))
 		exit(128);
 
+	diff_set_mnemonic_prefix(&revs->diffopt, "c/", cached ? "i/" : "w/");
 	diffcore_std(&revs->diffopt);
 	diff_flush(&revs->diffopt);
 	return 0;