builtin: patch-id: remove unused diff-tree prefix

The last git version that had "diff-tree" in the header text
of "git diff-tree" output was v1.3.0 from 2006. The header text
was changed from "diff-tree" to "commit" in 91539833
("Log message printout cleanups").

Given how long ago this change was made, it is highly unlikely that
anyone is still feeding in outputs from that git version.

Remove the handling of the "diff-tree" prefix and document the
source of the other prefixes so that the overall functionality
is more clear.

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

builtin/patch-id.c

@@ -74,8 +74,8 @@ static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 		const char *p = line;
 		int len;
 
-		if (!skip_prefix(line, "diff-tree ", &p) &&
-		    !skip_prefix(line, "commit ", &p) &&
+		/* Possibly skip over the prefix added by "log" or "format-patch" */
+		if (!skip_prefix(line, "commit ", &p) &&
 		    !skip_prefix(line, "From ", &p) &&
 		    starts_with(line, "\\ ") && 12 < strlen(line)) {
 			if (verbatim)