Merge branch 'cc/subprocess-handshake-missing-capabilities' into maint

When handshake with a subprocess filter notices that the process
asked for an unknown capability, Git did not report what program
the offending subprocess was running.  This has been corrected.

We may want a follow-up fix to tighten the error checking, though.

* cc/subprocess-handshake-missing-capabilities:
  sub-process: print the cmd when a capability is unsupported

sub-process.c

@@ -181,8 +181,8 @@ static int handshake_capabilities(struct child_process *process,
 			if (supported_capabilities)
 				*supported_capabilities |= capabilities[i].flag;
 		} else {
-			warning("external filter requested unsupported filter capability '%s'",
-				p);
+			warning("subprocess '%s' requested unsupported capability '%s'",
+				process->argv[0], p);
 		}
 	}