Patch from ch3cooli (modified by me):
If file is readonly, run "Save as" (ask new filename).
Especially for left view when edit mode is enabled, because there is no such checking.

git-svn-id: http://tortoisesvn.googlecode.com/svn/trunk/src/TortoiseMerge@24087 4cfb86a4-2c7a-11df-bd14-c1e331e6ceac