tests/test-hghave.t

fix: highlight the required configuration and behavior of the fixer tools

The problem is that `hg help fix` didn't mention *how* to configure the tools,
and while I knew that `{rootpath}` existed in the configuration, I missed that
the tools require reading content from stdin.  (I configured `gofmt` to use
`{rootpath}`, and that had the effect of squashing all changes in a file at `.`
into the first commit and emptying that content from its descendants.)

Basically all this does is put a pointer in the default (command level) help to
the extension level help that mentions the configuration, and moves the
extension level help that documents reading from stdin and writing to stdout to
the top to give it more prominence.  The last sentence is adjusted a bit to
reflect the new location.

  $ . "$TESTDIR/helpers-testrepo.sh"

Testing that hghave does not crash when checking features

  $ hghave --test-features 2>/dev/null

Testing hghave extensibility for third party tools

  $ cat > hghaveaddon.py <<EOF
  > import hghave
  > @hghave.check("custom", "custom hghave feature")
  > def has_custom():
  >     return True
  > EOF

(invocation via run-tests.py)

  $ cat > test-hghaveaddon.t <<EOF
  > #require custom
  >   $ echo foo
  >   foo
  > EOF
  $ ( \
  > testrepohgenv; \
  > "$PYTHON" $TESTDIR/run-tests.py --with-hg=$HGTEST_REAL_HG -j 1 \
  >    $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
  > )
  installed Mercurial in * seconds  (glob) (?)
  running 1 tests using 1 parallel processes 
  .
  # Ran 1 tests, 0 skipped, 0 failed.

(invocation via command line)

  $ unset TESTDIR
  $ hghave custom

(terminate with exit code 2 at failure of importing hghaveaddon.py)

  $ rm hghaveaddon.*
  $ cat > hghaveaddon.py <<NO_CHECK_EOF
  > importing this file should cause syntax error
  > NO_CHECK_EOF

  $ hghave custom
  failed to import hghaveaddon.py from '.': invalid syntax (hghaveaddon.py, line 1)
  [2]