ui: simplify geteditor Now $EDITOR and $VISUAL will affect ui.editor directly. So it's no longer necessary to test them in ui.geteditor.

debugconfig: list environment variables in debug output Since we print "read config from" for config files, printing environment variables will make it more consistent.

rcutil: let environ override system configs (BC) This is BC because system configs won't be able to override $EDITOR, $PAGER. The new behavior is arguably more rational.

rcutil: add a method to convert environment variables to config items Handling config and environ priorities has been messy. Partially because we don't have config layers - you either get all configs (sys + user), or none. Ideally, environ like $EDITOR, $PAGER should be able to override the system configs "ui.editor", "pager.pager". This patch provides the ability to convert them into config items, so they can be inserted into the middle config layer between system rc and user rc.

rcutil: let rccomponents return different types of configs (API) The next patches will convert environ to raw config items, and insert the config items between systemrcpath and userrcpath. This patch teaches rccomponents to return the type information so the caller could distinguish between "path" and raw config "items".

rcutil: rename rcpath to rccomponents (API)

rcutil: extract rc directory listing logic The logic of listing a ".rc" directory is duplicated in two functions, extract it to a single function to make the code cleaner.

rcutil: split osrcpath to return default.d paths (API) After this change, there are 3 rcpath functions: - defaultrcpath - systemrcpath - userrcpath This will allow us to insert another config layer in the middle.

rcutil: move scmutil.*rcpath to rcutil (API) As discussed at [1], the logic around "actual config"s seem to be non-trivial enough that it's worth a new module. This patch creates the module and move "scmutil.*rcpath" functions there as the first step. More methods will be moved to the module in the future. The module is different from config.py because the latter only cares about data structure and parsing, and does not care about special case, or system config paths, or environment variables. [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/095503.html

statfs: make getfstype() raise OSError It's better for getfstype() function to not suppress an error. Callers can handle it as necessary. Now "hg debugfsinfo" will report OSError.