The default branch detection added by commit 26ec2e2b0c (Tests: Update
CTest.UpdateGIT test for custom defaultBranch, 2020-12-02,
v3.20.0-rc1~355^2) fails if the test is run inside a Git work tree whose
`.git` directory configures a `defaultBranch` that is different from the
global or system-wide value.  Fix the detection to ignore the locally
configured value so that we match the `git init` decision.

Git installation now prompts the user to choose a system-wide
name for the default branch in new repositories.

Automate with:

find Tests -type f -print0 | xargs -0 perl -i -0pe \
's/set\(([a-zA-Z0-9_]+)(\s+)"\$\{\1\}([^"])/string(APPEND \1\2"\3/g'

Avoid constructing full paths to .git repositories in the test.  Use
relative paths and let Git convert them to absolute paths internally.
This is simpler and also avoids trouble with various absolute path root
component conventions on Windows (`c:/`, `/c/`, `/cygdrive/c/`).

Make RETURN_VALUE report -1 if the update command failed as the
documentation claims.  Also avoid reporting a ctest script-level failure
if the update command fails because we still correctly administered the
update step.

Parameterize the dashboard script generation to configure custom
content.  Also move state cleanup from previous test back to its block.

This allows ctest_update to get the current version without actually
changing the repository.  This is useful when using Jenkins or an
external project to update the source to a specific version, but you
still want the current version to show up in CDash.

Add the '--' command-line separator to make the HEAD reference
unambiguous.  Extend the CTest.UpdateGIT test to cover this case.

Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the
block.  This is no longer the preferred style.

Run the following shell code:

for c in else endif endforeach endfunction endmacro endwhile; do
    echo 's/\b'"$c"'\(\s*\)(.\+)/'"$c"'\1()/'
done >convert.sed &&
git ls-files -z -- bootstrap '*.cmake' '*.cmake.in' '*CMakeLists.txt' |
egrep -z -v '^(Utilities/cm|Source/kwsys/)' |
egrep -z -v 'Tests/CMakeTests/While-Endwhile-' |
xargs -0 sed -i -f convert.sed &&
rm convert.sed