apply upstream patch to resolve git fast-import memory leak (#1900335)
    
    A change in git-2.27.0¹ caused fast-import to leak memory and crash in
    some cases.  Apply the upstream fix², which didn't quite make it into
    git-2.29.0.
    
    ¹ ddddf8d7e2 (fast-import: permit reading multiple marks files, 2020-02-22)
      https://github.com/git/git/commit/ddddf8d7e2
    
    ² 3f018ec716 (fast-import: fix over-allocation of marks storage, 2020-10-15)
      https://github.com/git/git/commit/3f018ec716