OPT: Faster approach to GitRepo.format_commit()

[mih]

The previous git-show can be slow with more complex commits (e.g.
octopus merge commit, see gh-4801)

Switch to git-log as suggested in #4801.

Companion of #4806

[codecov]

Codecov Report

Merging #4807 into maint will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            maint    #4807      +/-   ##
==========================================
- Coverage   89.67%   89.65%   -0.02%     
==========================================
  Files         288      288              
  Lines       40352    40352              
==========================================
- Hits        36184    36179       -5     
- Misses       4168     4173       +5     

Impacted Files	Coverage Δ
datalad/support/gitrepo.py	90.37% <100.00%> (-0.08%)	⬇️
datalad/core/distributed/clone.py	88.62% <0.00%> (-0.60%)	⬇️
datalad/support/tests/test_annexrepo.py	96.59% <0.00%> (-0.16%)	⬇️

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[kyleam]

Looks good to me.

My only nit is that a comment might prevent that log from getting rewritten as show way down the road.

If anyone doesn't have an octopus merge at hand and wants to observe the slowness:

octopus script

set -eu
cd "$(mktemp -d "${TMPDIR:-/tmp}"/dl-XXXXXXX)"

git init
git commit --allow-empty -mc0

N=500
base=$(git symbolic-ref --short HEAD)
for i in $(seq 1 $N)
do
    git checkout -b b$i $base
    :>f$i
    git add f$i
    git commit -m"f$i"
done

git checkout $base
git merge -m"merge" $(seq --format="b%g" --separator=" " 1 $N)

[mih]

Thx @kyleam will add that comment

[mih]

Force-pushed missing comment. Thx @kyleam

[kyleam]

Thanks!