Handle early git version's lack of get-url

[zeripath]

Old versions of git do not have git remote get-url in which case we can use git config --get remote.<name>.url - however, please note that this is not a complete replacement for this as it will not handle insteadOf configuration. I do not think it is reasonable however to completely replicate this functionality and if users require that they should upgrade.

Fix #6930

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@d01e728). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #7065   +/-   ##
=========================================
  Coverage          ?   41.42%           
=========================================
  Files             ?      442           
  Lines             ?    59701           
  Branches          ?        0           
=========================================
  Hits              ?    24731           
  Misses            ?    31733           
  Partials          ?     3237

Impacted Files	Coverage Δ
models/repo_mirror.go	22.91% <0%> (ø)

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[lafriks]

Please backporr