Correctly append tags on patches without commit details

Correctly append tags on patches without commit details

Only a commit summary (a.k.a. patch subject) is necessary in Git: we
don't need details. The regex we were using to search for postscripts
however assumed that there would be _something_ before the postscript,
resulting in a newline. This wasn't the case. Correct this assumption by
instead using 're.MULTILINE' and matching on '^' and '$' for newlines
instead of '\n'.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #516
Cc: siddhesh@gotplt.org
(cherry picked from commit 05da32fef9140412bf0e822ce7f9706cb8aa7b5b)