git-mergetool: add new merge tool TortoiseMerge
TortoiseMerge comes with TortoiseSVN or TortoiseGit for Windows. It can
only be used as a merge tool with an existing base file. It cannot be
used without a base nor as a diff tool.
The documentation only mentions the slash '/' as command line option
prefix, which refused to work, but the parser also accepts the dash '-'
See http://code.google.com/p/msysgit/issues/detail?id=226
Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>