Allow to run git-extras within git-bulk

[guenhter]

When you execute

git bulk summary

it only summarizes the first found repo.
The reason for it seems to be some stdin ... read ... blah stuff (https://stackoverflow.com/a/13800476/5247292)

Therefore I changed the way the single repos of the bulk operation are worked off to avoid stdin when in the loop

Fix #1007

[spacewander]

It seems that the summary only shows part of the data. Running the command on my machine, got:

 git bulk -a summary
Core command "summary" accepted.
-> executing git summary

 project  : git-extras
 repo age : 12 years
 active   : 795 days
 commits  : 1688
 files    : 315
 authors  :

[guenhter]

Hm, interesting.

[guenhter]

Seems like I fall into the trap of git shortlog

If no revisions are passed on the command line and either standard input is not a terminal or there is no current branch, git shortlog will output a summary of the log read from standard input, without reference to the current repository.

I fixed that in the git-extras by passing always a reference to the git shortlog