makefile: Allow bypassing conflict check

[hyperupcall]

Introduces a SKIP_CONFLICT_CHECK so installs can always be unattended.

• Changes wording of conflicts with an alias, still install it and idsable the alias? [y/n]
  • Removes the part that says "disable the alias" because any existing aliases aren't (and shouldn't be) disabled
• Adds test to the .PHONY target
• Improves quoting for paths with spaces
• Improves variable names

And,

• Closes Install not possible in headless environment due to conflicting git alias #541
• Closes set-upstream utility #375 (stale, built into git)
• Closes Manpage for git-obliterate missing. #295 (stale, already fixed)
• Closes Show option: expand alias in terminal before it gets executed (like abbr -a in fish shell) #968 (stale, out of scope)

[spacewander]

Merged. Thanks!