git fast-import leaks memory drastically, so crashes with out of memory by attempt to import 22MB export dump

Steps to reproduce the issue:

  1. export from fossil and import the dump to the git:
```
fossil export --git --import-marks .git/.fossil2git-fssl --export-marks 
.git/.fossil2git-fssl.tmp ^
  | git fast-import --import-marks=.git/.fossil2git-git 
--export-marks=.git/.fossil2git-git.tmp
```
  during the import git-fast-import.exe is growing on memory (more than I 
have physically, e.g. noticed over 20GB),
  SO FINALLY IT IS CRASHING WITH:
```
fatal: Out of memory, malloc failed (tried to allocate 2097152 bytes)
fast-import: dumping crash report to .git/fast_import_crash_1800
```
  the crash report contains:
```
fast-import crash report:
  fast-import process: 1800
  parent process : 1
  at 2020-10-13 18:55:19 +0000

fatal: Out of memory, malloc failed (tried to allocate 2097152 bytes)

Most Recent Commands Before Crash ---------------------------------

Active Branch LRU
-----------------
  active_branches = 0 cur, 5 max

  pos clock name
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Inactive Branches -----------------

Marks
-----
  exported to .git/.fossil2git-git.tmp

------------------- END OF CRASH REPORT ```

  2. if I do the export firstly (redirect to file), fossil creates 22MB 
large export dump-file,
  if I import it via git hereafter, it crashes in the same way.
```
fossil export --git --import-marks .git/.fossil2git-fssl --export-marks 
.git/.fossil2git-fssl.tmp > tmp-dump-out-of-mem.txt

dir tmp-dump-out-of-mem.txt 13.10.2020 20:37 22.916.280 tmp-dump-out-of-mem.txt

type tmp-dump-out-of-mem.txt | git fast-import --import-marks=.git/.fossil2git-git --export-marks=.git/.fossil2git-git.tmp

fatal: Out of memory, malloc failed (tried to allocate 2097152 bytes) fast-import: dumping crash report to .git/fast_import_crash_1800 ```

I did not see any issues with (even much larger) imports, before I upgraded git to 2.28.0 (from 2.25.1, I guess).

[System Info] git version: git version 2.28.0.windows.1 cpu: x86_64 built from commit: 77982caf269b7ee713a76da2bcf260c34d3bf7a7 sizeof-long: 4 sizeof-size_t: 8 shell-path: /bin/sh uname: Windows 10.0 18363 compiler info: gnuc: 10.2 libc info: no libc information available $SHELL (typically, interactive shell): <unset>

An attempt to repeat this with 2.27.0 (portable) changes nothing (crashed with the same issue).

BUT TRYING THAT WITH 2.14.4 (MINGIT-2.14.4.WINDOWS.7-64-BIT) WORKS WELL:
```
C:SoftDevGit-2.14mingw64bingit-fast-import.exe statistics:
---------------------------------------------------------------------
Alloc'd objects: 130000
Total objects: 591 ( 80 duplicates )
  blobs : 224 ( 0 duplicates 151 deltas of 224 attempts)
  trees : 260 ( 80 duplicates 191 deltas of 260 attempts)
  commits: 107 ( 0 duplicates 0 deltas of 0 attempts)
  tags : 0 ( 0 duplicates 0 deltas of 0 attempts)
Total branches: 201 ( 46 loads )
  marks: 1048576 ( 129437 unique )
  atoms: 1503
Memory total: 10439 KiB
  pools: 4346 KiB
  objects: 6093 KiB
---------------------------------------------------------------------
pack_report: getpagesize() = 65536
pack_report: core.packedGitWindowSize = 1073741824
pack_report: core.packedGitLimit = 35184372088832
pack_report: pack_used_ctr = 131659
pack_report: pack_mmap_calls = 119
pack_report: pack_open_windows = 66 / 66
pack_report: pack_mapped = 488903419 / 488903419
---------------------------------------------------------------------
```

The crash with newer versions is pretty well reproducible, so I have frozen this state to be able to test it later (or check whether it gets fixed). Don't hesitate to ping me if you need some data or tests.