The "a..b" revision syntax was designed to handle commits,
so it doesn't bother to record any mode we find while
traversing a "tree:path" endpoint. These days "git diff" can
diff blobs using either "a:path..b:path" (with dots) or
"a:path b:path" (without), but the two behave
inconsistently, as the with-dots version fails to notice the
mode.
Let's teach the dot-dot range parser to record modes; it
doesn't cost us anything, and it makes this case work.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>