single-line output of external command to status bar

[rolandwalker]

Adds new external command option flag +, meaning "echo first line of the output to the status bar". Unlike !, output is integrated into the regular TUI and need not be manually dismissed.

Example usage:

# Show commit statistics for the author under the cursor
bind main U +sh -c 'git --no-pager shortlog -s --author="$(git show -s --format=%aE %(commit))" </dev/tty'

I think this is a good feature, but recognize that open_external_viewer() is taking too many params.

It might be better if the feature was "collapse whitespace and display as much output as fits in one screen width", though the user can get that by appending | xargs to a command.

(Odd little bug in git-shortlog: no output from the above command without </dev/tty.)

Edit: flagname +

[jonas]

Very nice. Didn't look yet, but from the description I'd prefer another symbol than _ because it is hard to read. Unfortunately < is used for quitting so maybe >?

Seems to fix #557 and #200.

[rolandwalker]

I actually wanted > for something else. Would % be okay? Or +?

[jonas]

Yes, + looks better. Although I am almost up for using < and finding another modifier for exiting.