allow configuration of character count in string diff formatting

[mitchdraft]

from format/format.go:

/*

Generates a nicely formatted matcher success / failure message

Much like Message(...), but it attempts to pretty print diffs in strings

Expected
    <string>: "...aaaaabaaaaa..."
to equal               |
    <string>: "...aaaaazaaaaa..."

*/

func MessageWithDiff(actual, message, expected string) string {

and https://github.com/onsi/gomega/blob/master/format/format.go#L146

we get 5 characters of context around the first diff in a string comparison assertion error message.

5 characters is often enough, however when comparing large structs, as can be the case with protobuf messages example it would be useful to have more context. Otherwise, long values can obscure the key, and make it harder to back-out the issue.

Suggestion: export format.CharactersAroundMismatchToInclude

[williammartin]

Thanks for the issue. Just to check, the format.TruncatedDiff value set to false isn't satisfactory for you here?

[mitchdraft]

Thanks for pointing that out, I hadn't considered it.
Still, I find the truncation super helpful on large structs. It has offered a nice balance of ease-of-test creation and usefulness of error messages.

In this util, for example. here are my notes on the alternatives - feedback welcome, maybe there's a better way

[mitchdraft]

additional note - we also decided to avoid MatchFields so as to avoid overlooking any new fields that might be added in the future

[blgm]

This was resolved by #347