range-diff: fix a crash in parsing git-log output

`git range-diff` calls `git log` internally and tries to parse its
output. But `git log` output can be customized by the user in their
git config and for certain configurations either an error will be
returned by `git range-diff` or it will crash.

To fix this explicitly set the output format of the internally
executed `git log` with `--pretty=medium`. Because that cancels
`--notes`, add explicitly `--notes` at the end.

Also, make sure we never crash in the same way - trying to dereference
`util` which was never created and has remained NULL. It would happen
if the first line of `git log` output does not begin with 'commit '.

Alternative considered but discarded - somehow disable all git configs
and behave as if no config is present in the internally executed
`git log`, but that does not seem to be possible. GIT_CONFIG_NOSYSTEM
is the closest to it, but even with that we would still read
`.git/config`.

Signed-off-by: Vasil Dimov <vd@FreeBSD.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -63,6 +63,8 @@ static int read_patches(const char *range, struct string_list *list,
 			"--output-indicator-old=<",
 			"--output-indicator-context=#",
 			"--no-abbrev-commit",
+			"--pretty=medium",
+			"--notes",
 			NULL);
 	if (other_arg)
 		argv_array_pushv(&cp.args, other_arg->argv);
@@ -106,6 +108,17 @@ static int read_patches(const char *range, struct string_list *list,
 			continue;
 		}
 
+		if (!util) {
+			error(_("could not parse first line of `log` output: "
+				"did not start with 'commit ': '%s'"),
+			      line);
+			string_list_clear(list, 1);
+			strbuf_release(&buf);
+			strbuf_release(&contents);
+			finish_command(&cp);
+			return -1;
+		}
+
 		if (starts_with(line, "diff --git")) {
 			struct patch patch = { 0 };
 			struct strbuf root = STRBUF_INIT;

t/t3206-range-diff.sh

@@ -513,6 +513,16 @@ test_expect_success 'range-diff overrides diff.noprefix internally' '
 	git -c diff.noprefix=true range-diff HEAD^...
 '
 
+test_expect_success 'basic with modified format.pretty with suffix' '
+	git -c format.pretty="format:commit %H%d%n" range-diff \
+		master..topic master..unmodified
+'
+
+test_expect_success 'basic with modified format.pretty without "commit "' '
+	git -c format.pretty="format:%H%n" range-diff \
+		master..topic master..unmodified
+'
+
 test_expect_success 'range-diff compares notes by default' '
 	git notes add -m "topic note" topic &&
 	git notes add -m "unmodified note" unmodified &&