tests(mingw): avoid very slow `mingw_test_cmp`

It is more performant to run `git diff --no-index` than running the
`mingw_test_cmp` code with MSYS2's Bash, i.e. the Bash that Git for
Windows uses. And a lot more readable.

The original reason why Git's test suite needs the `mingw_test_cmp`
function at all (and why `cmp` is not good enough) is that Git's test
suite is not actually trying to compare binary files when it calls
`test_cmp`, but it compares text files. And those text files can contain
CR/LF line endings depending on the circumstances.

Note: The original fix in the Git for Windows project implemented a test
helper that avoids the overhead of the diff machinery, in favor of
implementing a behavior that is more in line with what `mingw_test_cmp`
does now. This was done to minimize the risk in using something as
complex as the diff machinery to perform something as simple as
determining whether text output is identical to the expected output or
not. This approach has served Git for Windows well for years, but the
attempt to upstream this saw a lot of backlash and distractions during
the review, was disliked by the Git maintainer and was therefore
abandoned. For full details, see the thread at
https://lore.kernel.org/git/pull.1309.git.1659106382128.gitgitgadget@gmail.com/t

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

2 files changed, 1 insertions, 67 deletions

diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 796093a7b3..2acfd733e7 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1450,72 +1450,6 @@ test_skip_or_die () {
 	error "$2"
 }
 
-# The following mingw_* functions obey POSIX shell syntax, but are actually
-# bash scripts, and are meant to be used only with bash on Windows.
-
-# A test_cmp function that treats LF and CRLF equal and avoids to fork
-# diff when possible.
-mingw_test_cmp () {
-	# Read text into shell variables and compare them. If the results
-	# are different, use regular diff to report the difference.
-	local test_cmp_a= test_cmp_b=
-
-	# When text came from stdin (one argument is '-') we must feed it
-	# to diff.
-	local stdin_for_diff=
-
-	# Since it is difficult to detect the difference between an
-	# empty input file and a failure to read the files, we go straight
-	# to diff if one of the inputs is empty.
-	if test -s "$1" && test -s "$2"
-	then
-		# regular case: both files non-empty
-		mingw_read_file_strip_cr_ test_cmp_a <"$1"
-		mingw_read_file_strip_cr_ test_cmp_b <"$2"
-	elif test -s "$1" && test "$2" = -
-	then
-		# read 2nd file from stdin
-		mingw_read_file_strip_cr_ test_cmp_a <"$1"
-		mingw_read_file_strip_cr_ test_cmp_b
-		stdin_for_diff='<<<"$test_cmp_b"'
-	elif test "$1" = - && test -s "$2"
-	then
-		# read 1st file from stdin
-		mingw_read_file_strip_cr_ test_cmp_a
-		mingw_read_file_strip_cr_ test_cmp_b <"$2"
-		stdin_for_diff='<<<"$test_cmp_a"'
-	fi
-	test -n "$test_cmp_a" &&
-	test -n "$test_cmp_b" &&
-	test "$test_cmp_a" = "$test_cmp_b" ||
-	eval "diff -u \"\$@\" $stdin_for_diff"
-}
-
-# $1 is the name of the shell variable to fill in
-mingw_read_file_strip_cr_ () {
-	# Read line-wise using LF as the line separator
-	# and use IFS to strip CR.
-	local line
-	while :
-	do
-		if IFS=$'\r' read -r -d $'\n' line
-		then
-			# good
-			line=$line$'\n'
-		else
-			# we get here at EOF, but also if the last line
-			# was not terminated by LF; in the latter case,
-			# some text was read
-			if test -z "$line"
-			then
-				# EOF, really
-				break
-			fi
-		fi
-		eval "$1=\$$1\$line"
-	done
-}
-
 # Like "env FOO=BAR some-program", but run inside a subshell, which means
 # it also works for shell functions (though those functions cannot impact
 # the environment outside of the test_env invocation).
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 6db377f68b..4fab1c1984 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1721,7 +1721,7 @@ case $uname_s in
 	test_set_prereq SED_STRIPS_CR
 	test_set_prereq GREP_STRIPS_CR
 	test_set_prereq WINDOWS
-	GIT_TEST_CMP=mingw_test_cmp
+	GIT_TEST_CMP="GIT_DIR=/dev/null git diff --no-index --ignore-cr-at-eol --"
 	;;
 *CYGWIN*)
 	test_set_prereq POSIXPERM