chg: deduplicate error handling of ui.system() This moves 'onerr' handling from low-level util.system() to higher level, which seems better API separation.

chg: refactor ui.system() to be partly overridden Since fd598149112b changed the signature of ui.system(), chgui.system() should have been updated. This patch factors out the util.system() call so that chg can override how a shell command is executed.

color: have the 'ui' object carry the '_colormode' directly Before this changeset, the value was carried by the class to work around limitation of the extensions initialisation. Now that the initialisation is cleanly handled in 'dispatch', we can drop this work around.

color: move triggering of the initialisation logic in core We now run the color initialisation as part of the standard dispatch. This is opening the way for multiple cleanups since we now have access to the multiple 'ui' object and we'll be able to see difference between global and local config. This cleanup will arrive in later changesets. As a side effect, the '--color' flag is now working without the extension. Since we now properly initialize color for each ui idependently, we get a warning message twice.

color: add the definition of '--color' in core If we want to be able to move the initialisation in core, we need core to be aware of that '--color' flag at all time. So we now have the definition in core. That flag is currently unprocessed without the extensions (will be fixed soon). In addition the default value for this flag in core is 'never'. Enabling the extensions change that default value to 'auto'.

color: handle 'ui.plain()' directly in mode setup If 'ui.plain()' is set we should not colorize. We move that logic into the function that determine and setup the color mode. As all other code respect the resulting mode this will be equivalent.