error using `git mergetool --tool=meld`

[Matthew Groth <mgroth49@gmail.com>]

When using `git mergetool --tool=kdiff3`, it works fine. But with `git 
mergetool --tool=meld` I get this for every merge conflict:

```
Normal merge conflict for 'builders/pdfBuilder.py':
   {local}: created file
   {remote}: created file
Traceback (most recent call last):
   File "/usr/local/bin/meld", line 20, in <module>
     filename = arg.split('=')[1]
IndexError: list index out of range
builders/pdfBuilder.py seems unchanged.
```

And the contents of /usr/local/bin/meld are:

arglist = []
for arg in sys.argv[1:]:
     if arg.startswith('--output'):
         filename = arg.split('=')[1] ##### <<--------- line 20
         newArg = '--output=' + fix_abspath(filename)
     elif arg.startswith('-'):
         newArg = arg
     else:
         newArg = fix_abspath(arg)
     arglist.append(newArg)

I'm on OSX: 10.12.4

git --version
git version 2.12.2

This is my first post to the mailing list, sorry if the bug report is 
missing anything.