allow calling index.add, index.move and index.remove with single items

[MarcelBeining]

added testing for it
closes#813

[MarcelBeining]

hm guys, I am not that into testing to understand why the tests failed in some environments, maybebe you can help me. =/ thanks

The only thing I see is that in AppVeyor, the test fails when I try to remove the added file again from the index with
stderr: 'fatal: pathspec 'git\test\test_index.py' did not match any files
however why is this file there in some environments but not there in others?

[Byron]

Thanks a lot for your contribution! Indeed, tests usually fail for PRs, which doesn't make things easy for contributors - my apologies.
The new test added here seems to work, so I am optimistic it will be fine when merged.

There is some added code duplication which I believe can mostly be avoided, please see the line comments for details.

[codecov-io]

Codecov Report

Merging #840 into master will decrease coverage by 0.14%.
The diff coverage is 86.2%.

@@ Coverage Diff @@## master #840 +/- ##
==========================================
- Coverage 94.79% 94.64% -0.15% 
==========================================
Files 59 59 Lines 9600 9401 -199 ==========================================
- Hits 9100 8898 -202 - Misses 500 503 +3

Impacted Files	Coverage Δ
git/test/test_index.py	96.52% <100%> (-0.05%)	⬇️
git/index/base.py	94.11% <83.33%> (-0.6%)	⬇️
git/objects/tree.py	59.64% <0%> (-0.93%)	⬇️
git/index/util.py	90.69% <0%> (-0.61%)	⬇️
git/index/typ.py	96.82% <0%> (-0.58%)	⬇️
git/refs/log.py	93.84% <0%> (-0.53%)	⬇️
git/objects/base.py	95.83% <0%> (-0.33%)	⬇️
git/refs/tag.py	96.66% <0%> (-0.31%)	⬇️
git/test/lib/helper.py	91.86% <0%> (-0.28%)	⬇️
git/refs/reference.py	95.74% <0%> (-0.26%)	⬇️
... and 28 more

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[Byron]

Thanks a lot for your contribution thus far !
I am closing this PR due to prolonged inactivity. Please feel free to reopen it in case it can proceed.