Here's what I have in my Mercurial.ini ...
[extdiff]
cmd.bcomp = C:\Program Files\Beyond Compare 3\BCompare.exe
opts.bcomp = /leftreadonly
So, the extdiff extension is working fine except that even when one side of the comparison is my working directory, as in the case where I only give one revision argument, e.g. hg bcomp -r 25 to get a diff between rev25 and the working directory, it causes my diff tool, in this case BC3, to open up a folder comparison session comparing a snapshot of rev25 to a snapshot of the working directory, such as...
left: C:\Windows\Temp\extdiff.v20d13s\MyCode\
right: C:\Windows\Temp\extdiff.q78g269\MyCode\
Is there a way for me to tell it to diff against the actual live working directory on the right side so I can edit from inside the diff tool? This seems to work just fine through SVN's external diff functionality. I realize it will display a lot of orphans on the right --I'll be glad to suffer through having to filter those out.
I've never found a way to prevent extdiff from copying the changed files to the Temp directory, but if your diff tool allows in-place edits then Mercurial will copy the changed files back to the working directory when you exit the tool.
For example, using Beyond Compare, this is the relevant piece from my Mercurial.ini:
[extdiff]
cmd.bcomp = C:\Program Files\Beyond Compare 3\BCompare.exe
opts.bcomp =/expandall /solo /lro