Why my MsysGit bash can not use winmerge as difftool

Question

I tried to use winmerge as a difftool, but my git bash just can not use it. I tried to use follow cmds.

git config --replace --global diff.tool winmerge
git config --replace --global difftool.winmerge.cmd "winmerge.sh \"$LOCAL\" \"$REMOTE\""
git config --replace --global difftool.prompt false

and tried to write .gitconfig file directly. And I download the Git2.5 find out the winmerge file in mergetoo/,and copy it to my Git. After set the PATH,git difftool --tool-helpshow as

    'git difftool --tool=<tool>' may be set to one of the following:
                    vimdiff
                    vimdiff2
                    winmerge

    The following tools are valid, but not currently available:
                    araxis
                    bc3
                    codecompare
                    ...

Then I tried git difftool,the bash just use vimdiff,and I triedgit difftool -t winmerge(or WinMerge),the bash also use vimdiff actually.

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My efforts seems has been effective. I uninstalled the git-for-windows 1.9.5 and installed the git-for-windows 2.4.5,then I copied the winmerge file from git-master.zip which was downloaded form github to my git's folder../git-core/mergetool/.And checked $PATH. After those step, I typed git config merge.tool winmergeandgit config diff.tool winmerge. And git mergetool successfully started WinMerge, however git difftool still can not start WinMerge.

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Thus my attempts are not single,so I don't know which operation is the key point.