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This change adds the ability to generate stacktraces in Google Test on
both failures of assertions/expectations and on crashes. The
stacktrace support is conditionally available only when using Abseil
with Google Test.

To use this support, run the test under Bazel with a command like this:

bazel test --define absl=1 --test_env=GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1 //path/to/your:test

The "--define absl=1" part enables stacktraces on assertion/expectation
failures.

The "--test_env=GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" part enables
the signal handler that logs a stacktrace in the event of a crash
(this also requires the "--define absl=1" part). This is not the
default since it may interfere with existing tests.

Support bazel test filtering

GTEST_TEST_FILTER_ENV_VAR_ was used to specify an environment variable to obtain
the default test filter from. By default it was unset which broke
"--test_filter" for bazel. This CL eliminates GTEST_TEST_FILTER_ENV_VAR_ and
explicitly obtains the default test filter from the environment variable
TESTBRIDGE_TEST_ONLY if it exists.

ignore .md for appveyor builds

Reduce the number of strcmp calling while initialization

doc sync

Doc sync/internal

Sync with internal docs

When we do parallel test execution with a process for a test,
initialization of gtest become performance bottleneck when the test
binary contains many testcases.

Especially, some parameterlized test in chromium browser affected by
largely when address sanitizer is enabled.
Address sanitizer does not allow using optimized strcmp function and
test addition in parameterized test require lookup of test case using
strcmp.

This patch reduces the number of strcmp, it is called when registering
parameterized test. Using reverse iterator improves the time to find
registered tests in such case.

Some tests for chromium browser using address sanitizer finished 2x
faster with this patch.

doc sync

Sync with internal docs

Adjust documentation files and links.

Removed "Documentation.md" not adding value and not consitent with internal docs

Part of documentation rationalization

Rename AdvancedGuide.md to advanced.md and adjust the links. 
Part of documentation rationalization work

Fuchsia: Change fdio include path.

Remove launchpad dependency from Fuchsia.

Adress, #1616, add  printer for std::nullptr_t

Add printer for std::nullptr_t, addressing https://github.com/google/googletest/issues/1616

formatting changes

formatting changes 

Formatting changes

Remove unused variable in Fuchsia.