patchtests: introduce test_unset_prereq, for debugging

> While working on the --convert-graft-file test, I missed that I was
> relying on the GPG prereq, by using output of test cases that were only
> run under that prereq.
>
> For debugging, it was really convenient to force that prereq to be
> unmet, but there was no easy way to do that. So I came up with a way,
> and this patch reflects the cleaned-up version of that way.
>
> For convenience, the following two methods are now supported ways to
> pretend that a prereq is not met:
>
> 	test_set_prereq !GPG
>
> and
>
> 	test_unset_prereq GPG

> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
> Published-As: https://github.com/dscho/git/releases/tag/test-unset-prereq-v1
> Fetch-It-Via: git fetch https://github.com/dscho/git test-unset-prereq-v1
>  t/test-lib-functions.sh | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
> index 7d620bf2a9a..76cd6630f29 100644
> --- a/t/test-lib-functions.sh
> +++ b/t/test-lib-functions.sh
> @@ -278,8 +278,20 @@ write_script () {
>  # The single parameter is the prerequisite tag (a simple word, in all
>  # capital letters by convention).
>  
> +test_unset_prereq () {
> +	! test_have_prereq "$1" ||
> +	satisfied_prereq="${satisfied_prereq% $1 *} ${satisfied_prereq#* $1 }"
> +}
> +
>  test_set_prereq () {
> -	satisfied_prereq="$satisfied_prereq$1 "
> +	case "$1" in
> +	!*)
> +		test_unset_prereq "${1#!}"
> +		;;
> +	*)
> +		satisfied_prereq="$satisfied_prereq$1 "
> +		;;
> +	esac
>  }
>  satisfied_prereq=" "
>  lazily_testable_prereq= lazily_tested_prereq=
>
> base-commit: 1f1cddd558b54bb0ce19c8ace353fd07b758510d

> So I came up with a way,
> and this patch reflects the cleaned-up version of that way.
> 
> For convenience, the following two methods are now supported ways to
> pretend that a prereq is not met:
> 
> 	test_set_prereq !GPG
> 
> and
> 
> 	test_unset_prereq GPG

> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
> Published-As: https://github.com/dscho/git/releases/tag/test-unset-prereq-v1
> Fetch-It-Via: git fetch https://github.com/dscho/git test-unset-prereq-v1
>  t/test-lib-functions.sh | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)

> 
> diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
> index 7d620bf2a9a..76cd6630f29 100644
> --- a/t/test-lib-functions.sh
> +++ b/t/test-lib-functions.sh
> @@ -278,8 +278,20 @@ write_script () {
>  # The single parameter is the prerequisite tag (a simple word, in all
>  # capital letters by convention).
>  
> +test_unset_prereq () {
> +	! test_have_prereq "$1" ||
> +	satisfied_prereq="${satisfied_prereq% $1 *} ${satisfied_prereq#* $1 }"
> +}
> +
>  test_set_prereq () {
> -	satisfied_prereq="$satisfied_prereq$1 "
> +	case "$1" in
> +	!*)
> +		test_unset_prereq "${1#!}"
> +		;;
> +	*)
> +		satisfied_prereq="$satisfied_prereq$1 "
> +		;;
> +	esac
>  }
>  satisfied_prereq=" "
>  lazily_testable_prereq= lazily_tested_prereq=
> 
> base-commit: 1f1cddd558b54bb0ce19c8ace353fd07b758510d
> -- 
> 2.17.0.windows.1.36.gdf4ca5fb72a
> 

>> For convenience, the following two methods are now supported ways to
>> pretend that a prereq is not met:
>> 
>> 	test_set_prereq !GPG
>> 
>> and
>> 
>> 	test_unset_prereq GPG
>
> I'm not sure this is the right way to do this.
>
> I wanted to run the whole test suite with all GPG tests skipped the
> other day.  With this 'test_unset_prereq' I would have to modify all
> test scripts containing tests depending on the GPG prereq and add
> 'test_unset_prereq GPG', right?