Merge branch 'jk/difftool-command-not-found' into maint

"git difftool" by default ignores the error exit from the backend commands it spawns, because often they signal that they found differences by exiting with a non-zero status code just like "diff" does; the exit status codes 126 and above however are special in that they are used to signal that the command is not executable, does not exist, or killed by a signal. "git difftool" has been taught to notice these exit status codes. * jk/difftool-command-not-found: difftool: always honor fatal error exit codes