30min Script in git 2.7.4 takes 22+ hrs in git 2.9.3

Hi all:

The following script attempts to merge 4 git repos into one, maintaining tag and branch content (but not SHAs). Each original repo basically gets its own subfolder in the new one. Original repos are first rewritten to have their history think they always belonged in the target subfolder.

The problem: the script takes 30 minutes for one environment including git 2.7.4, and generates a repo of about 30mb. When run by a coworker using git 2.9.3, it takes 22+ hours and generates a 10gb repo.

Clearly something here is very wrong. Either there's a pretty horrible regression or my idea is a pretty bad one ;)

General process for the script:
  - check out 4 repos
  - rewrite their history so they always thought they were in a subfolder
  - copy these 4 rewritten folders to a temporary location
  - get a list of branches and tags for each of the 4 repos
  - initialize a new repo with a readme.md
  - for each unique tag
       - check the 4 rewritten / backed up repos for the tag
       - for each of the 4 rewritten repos:
            - if the tag exists in that repo, merge it into the new
repo in a test branch
           -  git pull --no-edit ../intermediate/oneRewrittenRepo    (SLOW PART)
        - save the tag
   - for each unique branch (same logic)

So... yeah... 30mb + 30 minutes -> 11gb + 22 hours somewhere between these two versions of git?

According to coworker:

during each pass of the Tags' loop it's sitting for a long time on:

 git pull --no-edit ../intermediate/webtools.common

which runs in its turn

git fetch --update-head-ok ../intermediate/webtools.common

which in its turn runs

git-upload-pack ../intermediate/webtools.common

Any ideas here are much appreciated =/

The Script in question is here:
   https://gist.github.com/robstryker/4854fc86ab3714a5e1af353b98cbc768