Do not write .gitignore if already exists

[gaborbernat]

This ensures that we do not overwrite an existing .gitignore when the
target directory is not created by virtualenv.

Resolves #1862.

[codecov]

Codecov Report

Merging #1866 into master will decrease coverage by 4.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1866      +/-   ##
==========================================
- Coverage   93.91%   89.71%   -4.21%     
==========================================
  Files          86       86              
  Lines        4159     4161       +2     
==========================================
- Hits         3906     3733     -173     
- Misses        253      428     +175     

Flag	Coverage Δ
#tests	89.71% <100.00%> (-4.21%)	⬇️

Impacted Files	Coverage Δ
src/virtualenv/create/creator.py	95.27% <100.00%> (-0.73%)	⬇️
src/virtualenv/util/subprocess/_win_subprocess.py	0.00% <0.00%> (-95.09%)	⬇️
src/virtualenv/util/path/_pathlib/via_os_path.py	0.00% <0.00%> (-91.35%)	⬇️
src/virtualenv/util/subprocess/__init__.py	88.88% <0.00%> (-11.12%)	⬇️
src/virtualenv/app_data/__init__.py	88.37% <0.00%> (-6.98%)	⬇️
src/virtualenv/util/path/_sync.py	86.20% <0.00%> (-6.90%)	⬇️
...ualenv/create/via_global_ref/builtin/pypy/pypy2.py	92.20% <0.00%> (-6.50%)	⬇️
...ualenv/create/via_global_ref/builtin/pypy/pypy3.py	94.87% <0.00%> (-5.13%)	⬇️
src/virtualenv/info.py	95.00% <0.00%> (-5.00%)	⬇️
src/virtualenv/util/path/_pathlib/__init__.py	37.50% <0.00%> (-4.17%)	⬇️
... and 3 more

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