exp run: failure with custom options in `.gitconfig`

[karajan1001]

Bug Report

Experiments always failed quickly after we set merge.ff to only. And queued experiments failed at start if we set rebase.autostash to true

Description

Reproduce

Add

[merge]
    ff = only

to .gitconfig make any experiment failed and

[rebase]
    autostash = true

make any queue tasks failed.

Expected

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 2.17.1.dev1+g31635b83
---------------------------------
Platform: Python 3.9.8 on macOS-12.5-arm64-arm-64bit
Supports:
	azure (adlfs = 2022.7.0, knack = 0.9.0, azure-identity = 1.10.0),
	gdrive (pydrive2 = 1.14.0),
	gs (gcsfs = 2022.7.1),
	hdfs (fsspec = 2022.7.1, pyarrow = 9.0.0),
	webhdfs (fsspec = 2022.7.1),
	http (aiohttp = 3.8.1, aiohttp-retry = 2.6.1),
	https (aiohttp = 3.8.1, aiohttp-retry = 2.6.1),
	s3 (s3fs = 2022.7.1, boto3 = 1.21.21),
	ssh (sshfs = 2022.6.0),
	oss (ossfs = 2021.8.0),
	webdav (webdav4 = 0.9.7),
	webdavs (webdav4 = 0.9.7)
Cache types: reflink, hardlink, symlink
Cache directory: apfs on /dev/disk3s1s1
Caches: local
Remotes: https
Workspace directory: apfs on /dev/disk3s1s1
Repo: dvc, git

Additional Information (if any):

[pmrowla]

There is an existing issue for setting ff-only https://github.com/iterative/dvc/issues/7675

These issues need to be resolved in scmrepo, basically we need to be able to provide our own gitconfig when doing experiments git operations (and force scmrepo to override the user's configuration)

[karajan1001]

OK, so the only new issue is rebase.autostash will cause queued tasks fail.