merge script: improve log message subject
- point out remote-tracking branches as "remote branch
'upstream/master'";
- avoid misleading log messages when a tag and branch
share a name.
This approximates the builtin merge command's behavior well
enough to pass the relevant tests.
Based roughly on v1.6.4.2~10^2 (merge: indicate remote tracking
branches in merge message, 2009-08-09) and v1.6.4.2~10^2~1 (merge: fix
incorrect merge message for ambiguous tag/branch, 2009-08-09).
Cc: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>