Restore old file merging algorithm

[jesseduffield]

The old file merging algorithm, while not as elegant as the new version, was more specific in ensuring the order of files were maintained. So I'm restoring it :)

Currently if you stage/unstage random files at master, they get shuffled around.

[codecov-io]

Codecov Report

Merging #279 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage    89.6%   89.62%   +0.02%     
==========================================
  Files           9        9              
  Lines        1558     1562       +4     
==========================================
+ Hits         1396     1400       +4     
  Misses        161      161              
  Partials        1        1

Impacted Files	Coverage Δ
pkg/commands/git.go	59.78% <100%> (+0.59%)	⬆️

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