git-p4: fix git-p4.mapUser on Windows

git-p4: fix git-p4.mapUser on Windows

When running git-p4 on Windows, with multiple git-p4.mapUser entries in
git config - no user mappings are applied to the generated repository.

Reproduction Steps:

1. Add multiple git-p4.mapUser entries to git config on a Windows
   machine
2. Attempt to clone a p4 repository

None of the user mappings will be applied.

This issue is actually caused by gitConfigList, using split(os.linesep)
to convert the output of git config --get-all into a list. On Windows,
os.linesep is equal to '\r\n' - however git.exe returns configuration
with a line seperator of '\n'.

This leads to the list returned by gitConfigList containing only one
element - which contains the full output of git config --get-all in
string form, which causes problems for the code introduced to
getUserMapFromPerforceServer in 10d08a149d ("git-p4: map a P4 user to
Git author name and email address", 2016-03-01)

This issue should be caught by the test introduced in 10d08a1, however
would require running on Windows to reproduce.

Using splitlines solves this issue, by splitting config on all
typical delimiters ('\n', '\r\n' etc.)

Signed-off-by: George Vanburgh <gvanburgh@bloomberg.net>
Reviewed-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>