External merge tool not launched when not connected to my working area network

I have configured external merge tool as TortoiseMerge. 

When I am connected to my working area network, the merging tool is run as it should and I can solve conflicts with TortoiseMerge.

when I am disconnnected to my working area network, TortoiseMerge is not run and the conflict can't be merged. 

 I think it is related to the path where the .gitconfig is saved.
The file is saved on M:, where M: is a connected drive to my job network \\worknetwork\user\username.
For information, here is some variables that define some path.
``` console
HOMEDRIVE=M:
HOMEPATH=\
HOMESHARE=\\worknetwork\user\username
```

I think that the bug can be fixed by setting the path where to save the .gitconfig file inside the sourcetree GUI.