extdiff: do single file diffs from the wc with no copy Extdiff was always making a temporary directory and copying files even when not required. This change makes extdiff avoid the copy when diffing a single file that lives in the wc. This lets external diff tools edit the working copy file directly. It also lets other extensions resuse the functions in extdiff and get in-place diffs.

extdiff: made it less chatty in non-verbose mode Made the status info only display in verbose mode since most hg commands aren't so chatty. This also makes it cleaner for other extensions to call extdiff.

extdiff: un-nested two functions The functions in extdiff that create temporary repo copies for are useful in other extensions, so the change moves them at the module level.