[PATCH] test-lib-functions: make 'test_cmp_rev' more informative on failure

["SZEDER Gábor" <szeder.dev@gmail.com>]

The 'test_cmp_rev' helper is merely a wrapper around 'test_cmp'
checking the output of two 'git rev-parse' commands, which means that
its output on failure is not particularly informative, as it's
basically two OIDs with a bit of extra clutter of the diff header, but
without any indication of which two revisions have caused the failure:

  --- expect.rev  2018-11-17 14:02:11.569747033 +0000
  +++ actual.rev  2018-11-17 14:02:11.569747033 +0000
  @@ -1 +1 @@
  -d79ce1670bdcb76e6d1da2ae095e890ccb326ae9
  +139b20d8e6c5b496de61f033f642d0e3dbff528d

It also pollutes the test repo with these two intermediate files,
though that doesn't seem to cause any complications in our current
tests (meaning that I couldn't find any tests that have to work around
the presence of these files by explicitly removing or ignoring them).

Enhance 'test_cmp_rev' to provide a more useful output on failure with
less clutter:

  error: two revisions point to different objects:
    'HEAD^': d79ce1670bdcb76e6d1da2ae095e890ccb326ae9
    'extra': 139b20d8e6c5b496de61f033f642d0e3dbff528d

Doing so is more convenient when storing the OIDs outputted by 'git
rev-parse' in a local variable each, which, as a bonus, won't pollute
the repository with intermediate files.

While at it, also ensure that 'test_cmp_rev' is invoked with the right
number of parameters, namely two.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/test-lib-functions.sh | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index d158c8d0bf..fc84db67a1 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -854,9 +854,23 @@ test_must_be_empty () {
 
 # Tests that its two parameters refer to the same revision
 test_cmp_rev () {
-	git rev-parse --verify "$1" >expect.rev &&
-	git rev-parse --verify "$2" >actual.rev &&
-	test_cmp expect.rev actual.rev
+	if test $# != 2
+	then
+		error "bug in the test script: test_cmp_rev requires two revisions, but got $#"
+	else
+		local r1 r2
+		r1=$(git rev-parse --verify "$1") &&
+		r2=$(git rev-parse --verify "$2") &&
+		if test "$r1" != "$r2"
+		then
+			cat >&4 <<-EOF
+			error: two revisions point to different objects:
+			  '$1': $r1
+			  '$2': $r2
+			EOF
+			return 1
+		fi
+	fi
 }
 
 # Print a sequence of integers in increasing order, either with
-- 
2.20.0.rc0.134.gf0022f8e60