extdiff: allow a preconfigured merge-tool to be invoked There are three ways to configure an extdiff tool: 1) cmd.tool = (/path/to/exe optional) 2) tool = (path/to/exe optional) 3) tool = sometool someargs Previously, if no executable is specified in the first two forms, the named tool must be in $PATH, or the invocation fails. Since the [merge-tools] section already has the path to the diff executable, and/or the registry keys to find the executable on Windows, reuse that configuration for forms 1 and 2 instead of failing. We already fallback to [diff-tools] and then [merge-tools] for program arguments if they aren't specified in the [extdiff] section. Since this additional lookup only occurs if an executable is not on the $PATH for the named tool, this is backwards compatible. For now, we assume the user knows what he is doing if a path is provided. This change allows a configuration file like this (assuming beyondcompare3 is configured in merge-tools), instead of hardcoding system specific a path: [extdiff] beyondcompare3 =