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doc string fix: hg cat and manifest default to current parent revision.

A simple testing framework

To run the tests, do:

cd tests/
python run-tests.py

This finds all scripts in the test directory named test-* and executes
them. The scripts can be either shell scripts or Python. Each test is
run in a temporary directory that is removed when the test is complete.

A test-<x> succeeds if the script returns success and its output
matches test-<x>.out. If the new output doesn't match, it is stored in
test-<x>.err.

There are some tricky points here that you should be aware of when
writing tests:

- hg commit and hg merge want user interaction

  for commit use -m "text"
  for hg merge, set HGMERGE to something noninteractive (like true or merge)

- changeset hashes will change based on user and date which make
  things like hg history output change

  use commit -m "test" -u test -d "1000000 0"

- diff and export may show the current time

  use -D/--nodates to strip the dates

- You can append your own hgrc settings to the file that the environment
  variable HGRCPATH points to. This file is cleared before running a test.

You also need to be careful that the tests are portable from one platform
to another.  You're probably working on Linux, where the GNU toolchain has
more (or different) functionality than on MacOS, *BSD, Solaris, AIX, etc.
While testing on all platforms is the only sure-fire way to make sure that
you've written portable code, here's a list of problems that have been
found and fixed in the tests.  Another, more comprehensive list may be
found in the GNU Autoconf manual, online here:

    http://www.gnu.org/software/autoconf/manual/html_node/Portable-Shell.html

sh:

The Bourne shell is a very basic shell.  /bin/sh on Linux is typically
bash, which even in Bourne-shell mode has many features that Bourne shells
on other Unix systems don't have (and even on Linux /bin/sh isn't
guaranteed to be bash).  You'll need to be careful about constructs that
seem ubiquitous, but are actually not available in the least common
denominator.  While using another shell (ksh, bash explicitly, posix shell,
etc.) explicitly may seem like another option, these may not exist in a
portable location, and so are generally probably not a good idea.  You may
find that rewriting the test in python will be easier.

- don't use pushd/popd; save the output of "pwd" and use "cd" in place of
  the pushd, and cd back to the saved pwd instead of popd.

- don't use math expressions like let, (( ... )), or $(( ... )); use "expr"
  instead.

grep:

- don't use the -q option; redirect stdout to /dev/null instead.

- don't use extended regular expressions with grep; use egrep instead, and
  don't escape any regex operators.

sed:

- make sure that the beginning-of-line matcher ("^") is at the very
  beginning of the expression -- it may not be supported inside parens.

echo:

- echo may interpret "\n" and print a newline; use printf instead if you
  want a literal "\n" (backslash + n).

false:

- false is guaranteed only to return a non-zero value; you cannot depend on
  it being 1.  On Solaris in particular, /bin/false returns 255.  Rewrite
  your test to not depend on a particular return value, or create a
  temporary "false" executable, and call that instead.

diff:

- don't use the -N option.  There's no particularly good workaround short
  of writing a reasonably complicated replacement script, but substituting
  gdiff for diff if you can't rewrite the test not to need -N will probably
  do.