merge/pull: extend merge.stat configuration variable to cover --compact-summary

Existing `merge.stat` configuration variable is a Boolean that
defaults to `true` to control `git merge --[no-]stat` behaviour.

Extend it to be "Boolean or text", that takes false, true, or
"compact", with the last one triggering the --compact-summary option
introduced earlier.  Any other values are taken as the same as true,
instead of signaling an error---it is not a grave enough offence to
stop their merge.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/merge.adoc

@@ -81,8 +81,18 @@ as `false`. Defaults to `conflict`.
 	attributes" in linkgit:gitattributes[5].
 
 `merge.stat`::
-	Whether to print the diffstat between `ORIG_HEAD` and the merge result
-	at the end of the merge.  True by default.
+	What, if anything, to print between `ORIG_HEAD` and the merge result
+	at the end of the merge.  Possible values are:
++
+--
+`false`;; Show nothing.
+`true`;; Show `git diff --diffstat --summary ORIG_HEAD`.
+`compact`;; Show `git diff --compact-summary ORIG_HEAD`.
+--
++
+but any unrecognised value (e.g., a value added by a future version of
+Git) is taken as `true` instead of triggering an error.  Defaults to
+`true`.
 
 `merge.autoStash`::
 	When set to `true`, automatically create a temporary stash entry

builtin/merge.c

@@ -673,8 +673,35 @@ static int git_merge_config(const char *k, const char *v,
 	}
 
 	if (!strcmp(k, "merge.diffstat") || !strcmp(k, "merge.stat")) {
-		show_diffstat = git_config_bool(k, v)
-			? MERGE_SHOW_DIFFSTAT : 0;
+		int val = git_parse_maybe_bool_text(v);
+		switch (val) {
+		case 0:
+			show_diffstat = 0;
+			break;
+		case 1:
+			show_diffstat = MERGE_SHOW_DIFFSTAT;
+			break;
+		default:
+			if (!strcmp(v, "compact"))
+				show_diffstat = MERGE_SHOW_COMPACTSUMMARY;
+			/*
+			 * We do not need to have an explicit
+			 *
+			 * else if (!strcmp(v, "diffstat"))
+			 *	show_diffstat = MERGE_SHOW_DIFFSTAT;
+			 *
+			 * here, because the catch-all uses the
+			 * diffstat style anyway.
+			 */
+			else
+				/*
+				 * A setting from a future?  It is not an
+				 * error grave enough to fail the command.
+				 * proceed using the default one.
+				 */
+				show_diffstat = MERGE_SHOW_DIFFSTAT;
+			break;
+		}
 	} else if (!strcmp(k, "merge.verifysignatures")) {
 		verify_signatures = git_config_bool(k, v);
 	} else if (!strcmp(k, "pull.twohead")) {

t/t7600-merge.sh

@@ -216,6 +216,22 @@ test_expect_success 'merge c0 with c1 with --ff-only' '
 	verify_head "$c1"
 '
 
+test_expect_success 'the same merge with merge.stat=diffstat' '
+	cat >expect <<-\EOF &&
+	Updating FROM..TO
+	Fast-forward
+	 file  | 2 +-
+	 other | 9 +++++++++
+	 2 files changed, 10 insertions(+), 1 deletion(-)
+	 create mode 100644 other
+	EOF
+
+	git reset --hard c0 &&
+	git -c merge.stat=diffstat merge c1 >out &&
+	sed -e "1s/^Updating [0-9a-f.]*/Updating FROM..TO/" out >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'the same merge with compact summary' '
 	cat >expect <<-\EOF &&
 	Updating FROM..TO
@@ -231,6 +247,36 @@ test_expect_success 'the same merge with compact summary' '
 	test_cmp expect actual
 '
 
+test_expect_success 'the same merge with compact summary' '
+	cat >expect <<-\EOF &&
+	Updating FROM..TO
+	Fast-forward
+	 file        | 2 +-
+	 other (new) | 9 +++++++++
+	 2 files changed, 10 insertions(+), 1 deletion(-)
+	EOF
+
+	git reset --hard c0 &&
+	git merge --compact-summary c1 >out &&
+	sed -e "1s/^Updating [0-9a-f.]*/Updating FROM..TO/" out >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'the same merge with merge.stat=compact' '
+	cat >expect <<-\EOF &&
+	Updating FROM..TO
+	Fast-forward
+	 file        | 2 +-
+	 other (new) | 9 +++++++++
+	 2 files changed, 10 insertions(+), 1 deletion(-)
+	EOF
+
+	git reset --hard c0 &&
+	git -c merge.stat=compact merge c1 >out &&
+	sed -e "1s/^Updating [0-9a-f.]*/Updating FROM..TO/" out >actual &&
+	test_cmp expect actual
+'
+
 test_debug 'git log --graph --decorate --oneline --all'
 
 test_expect_success 'merge from unborn branch' '