global: Specify `-c core.fsmonitor=` to git commands

[akinomyoga]

@calestyo I have applied the partial fix.

Could you test whether it works for each of the related plugins and themes?

• There are two changed completions: gh and hub
  • [ ] gh
  • [ ] hub
• There are eight themes
  • [ ] agnoster
  • [ ] cooperkid
  • [ ] cupcake
  • [ ] doubletime
  • [ ] hawaii50
  • [ ] mbriggs
  • [ ] rana
  • [ ] sexy
• There are also changes in the basic utilities in libg/git.sh and in lib/omb-prompt-base.sh. This would affect most of the themes. So we should also test other themes that use these utilities.