histedit: raise ImportError when demandloading is enabled

--HG--
branch : stable
doc: reword "config file" to "configuration file"

While we do use the abbreviation "config" elsewhere in the docs, the
phrase "configuration file" is always written out in full, except in
this location in phases.txt.

--HG--
branch : stable
doc: make it easier to read how to enable extensions

We tell people all the time that enabling extensions is not a scary
thing to do, but we don't make it easy enough for an absolute novice
to do so. When they see a suggestion to do "hg extfoo bar", the error
message tells them "see hg help extensions", but that help page
doesn't actually tell them where configuration files are.

Furthermore, the big warning about why extensions aren't enabled by
default should be pushed down a little bit. Most of the extensions
shipped by hg are not all that scary, and some very basic and useful
cosmetic extensions like graphlog, color, pager, and progress, should
be enabled for many hg users.

--HG--
branch : stable
commit: amending with --close-branch (issue3445)

You can't close a branch that hasn't got a head.
newbranch + commit --close-branch must fail
newbranch + commit + commit --amend --close-branch must fail

You must not be allowed to close a branch that is not defined.

--HG--
branch : stable
color: only provide the required opt to _modesetup
color: use pythons isinstance method instead of reprogramming it
run-tests: open hgrc file only for writing

The file is not read here. Opening with "w+" is unnecessary.
run-tests: report interrupted tests

When the test run is aborted, a message is printed for each interrupted test.
This is helpful when a test is hanging.

example failure message:
INTERRUPTED: /path/to/tests/test-example.t

The message can appear before or after the line with the number of tests
run-tests: when interrupted report time in failure message

When --time is specified, the interruption message of an interrupted test is
extended with the time the test has run

INTERRUPTED: /path/to/tests/test-example.t (after 513 seconds)
run-tests: always gather runtimes
run-tests: remove spurious disable of --time with --debug
run-tests: simplify interrupted message
merge with stable