Drop return value from parse_user()

In commit 936295c (Simplify commit and tag parsing, 2015-03-03), the
commit and tag parsing code was refactored. This broke tag messages in
ui-tag since the line after the tagger header was erroneously skipped.
Rework parse_user() and skip the line manually outside parse_user().

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>