Make fzf '--preview' argument overridable in environment

[carlfriedrich]

Check list

• [x] I have performed a self-review of my code
• [n/a] I have commented my code in hard-to-understand areas
• [n/a] I have made corresponding changes to the documentation

Description

Arguments to fzf are passed via the FZF_DEFAULT_OPTS environment variable, which can be overridden by a dedicated FORGIT_<cmd>_FZF_OPTS variable for each command separately. The --preview argument, however, was excluded from this mechanism and hard-coded to the fzf call, so that there was no chance overriding it. This patch moves the passing of this argument to the FZF_DEFAULT_OPTS variable for all commands, so that it can be overridden as well.

Fixes #185

Type of change

• [x] Bug fix
• [ ] New feature
• [ ] Refactor
• [ ] Breaking change
• [ ] Documentation change

Test environment

• Shell
  • [x] bash
  • [ ] zsh
  • [ ] fish
• OS
  • [x] Linux
  • [ ] Mac OS X
  • [ ] Windows
  • [ ] Others:

[cjappl]

This LGTM! Pinging @wfxr for a review

[wfxr]

:+1: LGTM!

@carlfriedrich Looks like it conflicts with the previous pr. Can you help resolve it?

[carlfriedrich]

@wfxr Thanks for checking this and approving my other PR! I have rebased this on the current master, so it's ready to be merged now as well.

[wfxr]

@carlfriedrich Thanks for your contribution!