Merge branch 'ps/t7800-variable-interpolation-fix'

Fix the way recently added tests interpolate variables defined
outside them, and document the best practice to help future
developers.

* ps/t7800-variable-interpolation-fix:
  t/README: document how to loop around test cases
  t7800: use single quotes for test bodies
  t7800: improve test descriptions with empty arguments

Author: gitster

t/README

@@ -724,6 +724,26 @@ The "do's:"
    Note that we still &&-chain the loop to propagate failures from
    earlier commands.
 
+ - Repeat tests with slightly different arguments in a loop.
+
+   In some cases it may make sense to re-run the same set of tests with
+   different options or commands to ensure that the command behaves
+   despite the different parameters. This can be achieved by looping
+   around a specific parameter:
+
+	for arg in '' "--foo"
+	do
+		test_expect_success "test command ${arg:-without arguments}" '
+			command $arg
+		'
+	done
+
+   Note that while the test title uses double quotes ("), the test body
+   should continue to use single quotes (') to avoid breakage in case the
+   values contain e.g. quoting characters. The loop variable will be
+   accessible regardless of the single quotes as the test body is passed
+   to `eval`.
+
 
 And here are the "don'ts:"
 

t/t7800-difftool.sh

@@ -93,42 +93,42 @@ test_expect_success 'difftool forwards arguments to diff' '
 
 for opt in '' '--dir-diff'
 do
-	test_expect_success "difftool ${opt} ignores exit code" "
+	test_expect_success "difftool ${opt:-without options} ignores exit code" '
 		test_config difftool.error.cmd false &&
 		git difftool ${opt} -y -t error branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} forwards exit code with --trust-exit-code" "
+	test_expect_success "difftool ${opt:-without options} forwards exit code with --trust-exit-code" '
 		test_config difftool.error.cmd false &&
 		test_must_fail git difftool ${opt} -y --trust-exit-code -t error branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} forwards exit code with --trust-exit-code for built-ins" "
+	test_expect_success "difftool ${opt:-without options} forwards exit code with --trust-exit-code for built-ins" '
 		test_config difftool.vimdiff.path false &&
 		test_must_fail git difftool ${opt} -y --trust-exit-code -t vimdiff branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} honors difftool.trustExitCode = true" "
+	test_expect_success "difftool ${opt:-without options} honors difftool.trustExitCode = true" '
 		test_config difftool.error.cmd false &&
 		test_config difftool.trustExitCode true &&
 		test_must_fail git difftool ${opt} -y -t error branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} honors difftool.trustExitCode = false" "
+	test_expect_success "difftool ${opt:-without options} honors difftool.trustExitCode = false" '
 		test_config difftool.error.cmd false &&
 		test_config difftool.trustExitCode false &&
 		git difftool ${opt} -y -t error branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} ignores exit code with --no-trust-exit-code" "
+	test_expect_success "difftool ${opt:-without options} ignores exit code with --no-trust-exit-code" '
 		test_config difftool.error.cmd false &&
 		test_config difftool.trustExitCode true &&
 		git difftool ${opt} -y --no-trust-exit-code -t error branch
-	"
+	'
 
-	test_expect_success "difftool ${opt} stops on error with --trust-exit-code" "
-		test_when_finished 'rm -f for-diff .git/fail-right-file' &&
-		test_when_finished 'git reset -- for-diff' &&
+	test_expect_success "difftool ${opt:-without options} stops on error with --trust-exit-code" '
+		test_when_finished "rm -f for-diff .git/fail-right-file" &&
+		test_when_finished "git reset -- for-diff" &&
 		write_script .git/fail-right-file <<-\EOF &&
 		echo failed
 		exit 1
@@ -138,19 +138,19 @@ do
 		test_must_fail git difftool ${opt} -y --trust-exit-code \
 			--extcmd .git/fail-right-file branch >actual &&
 		test_line_count = 1 actual
-	"
+	'
 
-	test_expect_success "difftool ${opt} honors exit status if command not found" "
+	test_expect_success "difftool ${opt:-without options} honors exit status if command not found" '
 		test_config difftool.nonexistent.cmd i-dont-exist &&
 		test_config difftool.trustExitCode false &&
-		if test "${opt}" = '--dir-diff'
+		if test "${opt}" = --dir-diff
 		then
 			expected_code=127
 		else
 			expected_code=128
 		fi &&
-		test_expect_code \${expected_code} git difftool ${opt} -y -t nonexistent branch
-	"
+		test_expect_code ${expected_code} git difftool ${opt} -y -t nonexistent branch
+	'
 done
 
 test_expect_success 'difftool honors --gui' '