Truncate very long command lines when displaying them as an external source of data.

diffoscope/difference.py

@@ -280,7 +280,7 @@ class Difference:
 
         if 'source' not in kwargs:
             source_cmd = command1 or command2
-            kwargs['source'] = source_cmd.shell_cmdline()
+            kwargs['source'] = source_cmd.shell_cmdline(truncate=120)
 
         try:
             difference = Difference.from_feeder(

diffoscope/utils.py

@@ -18,7 +18,7 @@
 # along with diffoscope.  If not, see <https://www.gnu.org/licenses/>.
 
 
-def format_cmdline(cmd, replace=()):
+def format_cmdline(cmd, replace=(), truncate=None):
     def fn(x):
         if x in replace:
             return '{}'
@@ -27,4 +27,9 @@ def format_cmdline(cmd, replace=()):
             x = x[1:-1]
         return x
 
-    return ' '.join(fn(x) for x in cmd)
+    result = ' '.join(fn(x) for x in cmd)
+
+    if truncate is not None and len(result) > truncate:
+        result = result[: truncate + 4] + " […]"
+
+    return result