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markers from GDB will always be received in one chunk of input;
gud-gdb-marker-filter may be called several times, each time
providing a little more of the position marker.
(gud-gdb-marker-acc): New variable.
(gud-gdb-marker-filter): If we have received what could be the
beginning of a position marker, hold that text in
gud-gdb-marker-acc for the next time we get called, until we have
enough information to decide for sure.

* gud.el (gud-gdb-marker-filter): Only recognize GDB position
markers if they occur at the beginning of the line.  They always
do, and this reduces the likelihood that the above change will
hold back output that isn't really a position marker.