Add --from-history flag

[alicederyn]

It would be very convenient for rex to offer a --from-history flag, where it captures all the current shell's history, back to whatever the last git command was, and drop into an editor for a commit message with said commands listed in a script block.

This feature is likely to have a lot of subtle complexities to worry about; for instance, certain git commands like diff are a common part of establishing that a command has done what the user anticipated, so should be skipped over in the history.

Should respect the core.editor config, if set.

[alicederyn]

Note that I'm not sure if this feature is actually possible (i.e. whether a script can access the history of the invoking shell). Experimentation suggests a bash script can access its parent's history, but I suspect this is because it is run within the same process for efficiency; calling into bash via a python subprocess call returns a blank history.

It would be possible to grab commands from $HISTFILE (as an undocumented feature since by default it is not updated until the shell is closed), but they will be interleaved with commands from other shells in a way we cannot untangle.

[alicederyn]

Closing as this feature appears to be impossible to implement cleanly.