builtin-am: don't parse mail when resuming

builtin-am: don't parse mail when resuming

Since 271440e (git-am: make it easier after fixing up an unapplicable
patch., 2005-10-25), when "git am" is run again after being paused, the
current mail message will not be re-parsed, but instead the contents of
the state directory's patch, msg and author-script files will be used
as-is instead.

Re-implement this in builtin/am.c.

Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>