fix source buffer erroneously receiving focus

-after running commands from the source buffer (e.g. via Short-Key
mode) the source buffer always receives point focus following a
command, regardless of where that command came from. this makes
executing concurrent commands from the command buffer very tedious

-it appears 'realgud-cmd-remap' is only called when executing
commands from the source buffer, but this function is the only place
where 'in-srcbuf' (which ultimately determines which buffer obtains
focus) is set:

(realgud-cmdbuf-info-in-srcbuf?= (not (realgud-cmdbuf? buffer)))

hence 'in-srcbuf' is only ever set 't' from its initial 'nil' value,
resulting in the behaviour described

-this commit simply resets the 'in-srcbuf' variable at an appropriate
point in the command execution stack