BF(?WIN): provide relpath relative to dataset not default CWD

[yarikoptic]

Trying to address the #2 fail listed in #6237 and happening somehow only on master . Due to absent comments I assume that relpath was used to provide a shortened path to the message file. Given that dataset could point somewhere else (including another drive on windows) and os.curdir is used by relpath if source is not specified , I decided to just provide start to be the dataset path explicitly.
I failed to reproduce #6237 locally, so it is somewhat a wild shot, but what could go wrong? only reporting could be effected.

[mih]

Thx!

I cannot submit a proper suggesting, so doing it like this:

was:

    msg_path = opj(str(repo.dot_git), "COMMIT_EDITMSG")
    # shorten to the relative path for a more concise message
    msg_path = relpath(msg_path, ds_path)
    with open(msg_path, "wb") as ofh:
        ofh.write(ensure_bytes(msg))
    lgr.info("The command had a non-zero exit code. "
                "If this is expected, you can save the changes with "
                "'datalad save -d . -r -F %s'",
                msg_path)

could be

    msg_path = repo.dot_git / "COMMIT_EDITMSG"
    msg_path.write_text(msg)
    lgr.info("The command had a non-zero exit code. "
                "If this is expected, you can save the changes with "
                "'datalad save -d . -r -F %s'",
                # shorten to the relative path for a more concise message
                msg_path.relative_to(ds.pathobj)

[yarikoptic]

thanks @mih, applied

[codecov]

Codecov Report

Merging #6352 (c406a2b) into master (f3cbb0c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6352      +/-   ##
==========================================
- Coverage   89.84%   89.84%   -0.01%     
==========================================
  Files         331      331              
  Lines       43207    43206       -1     
==========================================
- Hits        38819    38818       -1     
  Misses       4388     4388              

Impacted Files	Coverage Δ
datalad/core/local/run.py	97.80% <100.00%> (-0.01%)	⬇️

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[yarikoptic]

ok, changed code is tests covered (not sure how much checked) and I took recommended changes. Let's proceed and improve/fix if needed after. Cheers!