mergetools: teach tortoisemerge to handle filenames with SP correctly

mergetools: teach tortoisemerge to handle filenames with SP correctly

TortoiseGitMerge, unlike TortoiseMerge, can be told to handle paths
with spaces in them by using -option "$FILE" (not -option:"$FILE",
which does not work for such paths) syntax.

This change was necessary because of MSYS path mangling [1], the ":"
after the "base" etc. arguments to TortoiseMerge caused the whole
argument instead of just the file name to be quoted in case of file
names with spaces. So TortoiseMerge was passed

    "-base:new file.txt"

instead of

    -base:"new file.txt"

(including the quotes). To work around this, TortoiseGitMerge does not
require the ":" after the arguments anymore which fixes handling file
names with spaces [2] (as written above).

[1] http://www.mingw.org/wiki/Posix_path_conversion
[2] https://github.com/msysgit/msysgit/issues/57

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Reported-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>