[PATCH 3/3] format-patch: use 'origin' as start of current-series-range when known

[Eric Sunshine <sunshine@sunshineco.com>]

When formatting a patch series over `origin..HEAD`, one would expect
that range to be used as the current-series-range when computing a
range-diff between the previous and current versions of a patch series.
However, infer_range_diff_ranges() ignores `origin..HEAD` when
--range-diff=<prev> specifies a single revision rather than a range, and
instead unexpectedly computes the current-series-range based upon
<prev>. Address this anomaly by unconditionally using `origin..HEAD` as
the current-series-range regardless of <prev> as long as `origin` is
known, and only fall back to basing current-series-range on <prev> when
`origin` is not known.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---

Notes:
    Suggested by Junio[1] during review.
    
    I had some difficulty composing the commit message and am not
    convinced that it does a good job of explaining the change or
    justifying it. If anyone can suggest improvements, I'd appreciate
    the help.
    
    I'm also not sure if this change deserves a test. If it does, it
    isn't clear to me exactly how to craft one. So, again, assistance
    would be appreciated.
    
    [1]: https://lore.kernel.org/git/xmqqva93t4u7.fsf@gitster-ct.c.googlers.com/

 builtin/log.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/builtin/log.c b/builtin/log.c
index 37177b3e7f..f79b2b8775 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1595,16 +1595,20 @@ static void infer_range_diff_ranges(struct strbuf *r1,
 				    struct commit *head)
 {
 	const char *head_oid = oid_to_hex(&head->object.oid);
+	int prev_is_range = !!strstr(prev, "..");
 
-	if (!strstr(prev, "..")) {
+	if (prev_is_range)
+		strbuf_addstr(r1, prev);
+	else
 		strbuf_addf(r1, "%s..%s", head_oid, prev);
+
+	if (origin)
+		strbuf_addf(r2, "%s..%s", oid_to_hex(&origin->object.oid), head_oid);
+	else if (prev_is_range)
+		die(_("failed to infer range-diff origin of current series"));
+	else {
+		warning(_("using '%s' as range-diff origin of current series"), prev);
 		strbuf_addf(r2, "%s..%s", prev, head_oid);
-	} else if (!origin) {
-		die(_("failed to infer range-diff ranges"));
-	} else {
-		strbuf_addstr(r1, prev);
-		strbuf_addf(r2, "%s..%s",
-			    oid_to_hex(&origin->object.oid), head_oid);
 	}
 }
 
-- 
2.28.0.618.gf4bc123cb7