[RFC] ripgrep: expose custom ignore files via --ignore-file-name

[akarle]

The ignore crate has the concept of "custom ignore" files. These files
take precedence over all others and can be used to declare arbitrary
file names as files which should be parsed for ignore patterns.

This commit adds the --ignore-file-name switch to ripgrep, exposing this
functionality to the user. For instance, if a user wanted to put their
ignores in many files called .my_ignore they could:

rg --ignore-file-name .my_ignore

This can also be used as a hack to honor .gitignore files outside .git
repositories (#1414), which can be useful if a git repo has been copied
somewhere without the .git directory (i.e. downloaded as a zip from
GitHub) or for SCM systems like Perforce which allow .gitignore files
but are not valid git repositories. Note, however, that this tactic
gives .gitignore precedence over .ignore and .rgignore, so consider
using multiple flags:

rg --ignore-file-name .ignore --ignore-file-name .gitignore

---

Aside: this is discussed in #1414 .

@BurntSushi : I have another branch on my fork, ignore_outside, that implements the other possible way we discussed of solving this (via a --force-gitignore flag, which I called --ignore-outside-git, but am happy to change to whatever we find appropriate).

I also came up with a third solution that would work for our use-case: creating a --alt-git-root-file switch which specifies a token that is determined to indicate the root of a git repo. This is the most intrusive implementation though (getting into the guts of the ignore crate), and I found my knowledge of asynchronous rust didn't cut it :)

I'm proposing we merge the --ignore-file-name switch, even though it is the most hacky solution to #1414 , because it is the least change to the code, and could have other benefits / be considered a feature. However, I'm happy to go with either the --ignore-outside-git or --alt-git-repo-files solutions if you deem them more appropriate for the project.

Rust is not my strongest language, so I appreciate any and all pointers / tips / corrections!

Thanks for your time and help!
Alex

[BurntSushi]

Thanks for this PR! The idea was interesting, but I think a more explicit --no-require-git flag is probably better. I've added this flag in #1486, which will be in the next release.

[akarle]

@BurntSushi -- thank you so much for taking the time to revisit this!

I agree, the --no-require-git flag is more explicit (and less likely to cause confusion). Thank you for implementing it! Looking forward to using it in the next release.