fmt-merge-msg: add '--(no-)log' options and 'merge.log' config variable

These are new synonyms to the '--(no-)summary' option and the
'merge.summary' config variable, but are consistent with the soon to be
added 'merge --(no-)log' options.  The 'merge.summary' config variable and
'--(no-)summary' options are still accepted, but are advertised to be
removed in the future.

'merge.log' takes precedence over 'merge.summary' if they are both set
inconsistently.

Update documentation and tests accordingly.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-fmt-merge-msg.c b/builtin-fmt-merge-msg.c
index ebb3f37..d49f545 100644
--- a/builtin-fmt-merge-msg.c
+++ b/builtin-fmt-merge-msg.c
@@ -6,13 +6,18 @@
 #include "tag.h"
 
 static const char *fmt_merge_msg_usage =
-	"git-fmt-merge-msg [--summary] [--no-summary] [--file <file>]";
+	"git-fmt-merge-msg [--log] [--no-log] [--file <file>]";
 
 static int merge_summary;
 
 static int fmt_merge_msg_config(const char *key, const char *value)
 {
-	if (!strcmp("merge.summary", key))
+	static int found_merge_log = 0;
+	if (!strcmp("merge.log", key)) {
+		found_merge_log = 1;
+		merge_summary = git_config_bool(key, value);
+	}
+	if (!found_merge_log && !strcmp("merge.summary", key))
 		merge_summary = git_config_bool(key, value);
 	return 0;
 }
@@ -250,9 +255,10 @@
 	git_config(fmt_merge_msg_config);
 
 	while (argc > 1) {
-		if (!strcmp(argv[1], "--summary"))
+		if (!strcmp(argv[1], "--log") || !strcmp(argv[1], "--summary"))
 			merge_summary = 1;
-		else if (!strcmp(argv[1], "--no-summary"))
+		else if (!strcmp(argv[1], "--no-log")
+				|| !strcmp(argv[1], "--no-summary"))
 			merge_summary = 0;
 		else if (!strcmp(argv[1], "-F") || !strcmp(argv[1], "--file")) {
 			if (argc < 3)