contrib: drop python 3.7 from the Windows dependency installer script I'm tempted to drop 3.8 too, since we use 3.9 on Windows.

contrib: drop python 3.5 requirements file for Linux automation The new minimum this cycle is py3.8.

extdiff: don't run gui programs when in a cli-only environment In order to provide a useful error message to override the behavior, we also need to slightly change the way that tool.gui is found in the config. Before, it had to be where tool.diffargs is located, which might not exist. Now, tool.isgui can exist on its own. A test is added for the new error message. We also need to force procutil.isgui() to return true, so we set $DISPLAY to a non-empty value before running any test expecting to have a gui.

bookflow: fix bullet list indentation in docstring

doc: generate separate commands/topics/extension pages This change modifies gendoc.py and Makefile so that individual pages for commands, help topics, and extensions can be generated. A new index page is also generated with links to all these pages. This makes it easier to look up and search the help text of a given command or topic, instead of having to deal with the giant hg.1 "all-in-one" page. Since the list of individual pages varies based on the source code, we generate a dynamic Makefile that contains this list of files as individual targets. This gives us fine-grained control over output files. However, it greatly increases the time spent generating all help pages. It's recommended to run make with -j to make use of multi-core archs. Individual man pages are produced in doc/man, and HTML ones are in doc/html

doc: refactor gendoc for better reusability This change separates the gathering of commands/topics/etc from the logic of printing their documentation out.

stream: prefer keeping an open file handle to volatile file instead of copy We will still do copy if too many file handle are open. Currently, have less than 10 volatile files in typical usage, so we should be fine. See inline documentation for details.

stream: remove __getitem__ from the VolatileManager It is no longer used and that API will get in the way of the new order.

stream: open volatile file through the manager To do more subtle things, we need more control.

stream: rename TempCopyManager to VolatileManager We are going to be more subtle in our management of volatile file, so use a more semantic name.