shortuser should stop before the first space character.
Examples:
"Foo Bar <foo.bar AT example DOT com>" -> "foo.bar"
(instead of "foo.bar AT example DOT com>" as it was before.)
"Foo Bar" -> "Foo"
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file named 'a' already in dirstate
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory named 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory named 'b' already in dirstate