[RFC/WIP] range-diff: show old/new blob OIDs in comments

[Eric Wong <e@80x24.org>]

(WIP, mostly stream-of-concious notes + reasoning)

When using "git format-patch --range-diff", the pre and
post-image blob OIDs are in each email, while the exact
commit OIDs are rarely shared via emails (only the tip
commit from "git request-pull").

These blob OIDs make it easy to search for or lookup the
full emails which create them, or the blob itself once
it's fetched via git.

public-inbox indexes and allows querying specifically for blob
OIDs via dfpre:/dfpost: since June 2017.  As of Jan 2019,
public-inbox also supports recreating blobs out of patch emails
(querying internally with dfpre:/dfpost: and doing "git apply")

Searching on these blob OIDs also makes it easier to find
previous versions of the patch sets using any mail search
engine.

Future changes to public-inbox may allow generating custom
diffs out of any blobs it can find or recreate.

Most of this is pretty public-inbox-specific and would've
made some future changes to public-inbox much easier....
(if we had this from the start of range-diff).

Unfortunately, it won't help with cases where range-diffs
are already published, but range-diff isn't too old.

I'm also still learning my way around git's C internals, but
using patch.{old,new}_oid_prefix seems alright...

FIXME: tests, t3206 needs updating

Not-signed-off-by: Eric Wong <e@80x24.org>
---
 range-diff.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/range-diff.c b/range-diff.c
index 7fed5a3b4b..85d2f1f58f 100644
--- a/range-diff.c
+++ b/range-diff.c
@@ -118,13 +118,24 @@ static int read_patches(const char *range, struct string_list *list)
 				die(_("could not parse git header '%.*s'"), (int)len, line);
 			strbuf_addstr(&buf, " ## ");
 			if (patch.is_new > 0)
-				strbuf_addf(&buf, "%s (new)", patch.new_name);
+				strbuf_addf(&buf, "%s (new %s)",
+						patch.new_name,
+						patch.new_oid_prefix);
 			else if (patch.is_delete > 0)
-				strbuf_addf(&buf, "%s (deleted)", patch.old_name);
+				strbuf_addf(&buf, "%s (deleted %s)",
+						patch.old_name,
+						patch.old_oid_prefix);
 			else if (patch.is_rename)
-				strbuf_addf(&buf, "%s => %s", patch.old_name, patch.new_name);
+				strbuf_addf(&buf, "%s => %s (%s..%s)",
+						patch.old_name,
+						patch.new_name,
+						patch.old_oid_prefix,
+						patch.new_oid_prefix);
 			else
-				strbuf_addstr(&buf, patch.new_name);
+				strbuf_addf(&buf, "%s (%s..%s)",
+						patch.new_name,
+						patch.old_oid_prefix,
+						patch.new_oid_prefix);
 
 			free(current_filename);
 			if (patch.is_delete > 0)