Use fish custom git command completion support

[guyboltonking]

Possibly fixes #697; other testers welcome! Since fish-shell/fish-shell#7652, fish has automatically discovered custom git commands and sourced the completion for them. This means that workarounds like

git-machete/completion/git-machete.fish

Lines 3 to 7 in 0d841cf

	# Caveat:
	# > if the user types git-machete, then only git-machete.fish will be loaded, but you can trick
	# > fish into still loading git.fish by adding a complete --do-complete="git " at the top
	# - https://gitter.im/fish-shell/fish-shell?at=6176a4bacd4972068b6a0804
	complete --do-complete="git "

...are no longer necessary.

[PawelLipski]

(I've changed the PR base to develop from master)

[codecov-commenter]

Codecov Report

Merging #993 (02f7799) into develop (7c4770e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #993   +/-   ##
========================================
  Coverage    98.50%   98.50%           
========================================
  Files           12       12           
  Lines         3147     3147           
  Branches       703      703           
========================================
  Hits          3100     3100           
  Misses          29       29           
  Partials        18       18           

[PawelLipski]

Gr8 🎉 I'm gonna release that as v3.17.7, and then add git machete completion bash|zsh|fish command in a subsequent v3.18.0 release 📦