lavf/concatdec: allow probing.

doc/demuxers.texi

@@ -44,8 +44,6 @@ length.
 
 All files must have the same streams (same codecs, same time base, etc.).
 
-This script format can currently not be probed, it must be specified explicitly.
-
 @subsection Syntax
 
 The script is a text file in extended-ASCII, with one directive per line.
@@ -62,6 +60,10 @@ backslash or single quotes.
 Identify the script type and version. It also sets the @option{safe} option
 to 1 if it was to its default -1.
 
+To make FFmpeg recognize the format automatically, this directive must
+appears exactly as is (no extra space or byte-order-mark) on the very first
+line of the script.
+
 @end table
 
 @subsection Options

libavformat/concatdec.c

@@ -40,7 +40,8 @@ typedef struct {
 
 static int concat_probe(AVProbeData *probe)
 {
-    return 0;
+    return memcmp(probe->buf, "ffconcat version 1.0", 20) ?
+           0 : AVPROBE_SCORE_MAX;
 }
 
 static char *get_keyword(uint8_t **cursor)