grepdiff1 test fails on musl

[alyssais]

musl does not accept the regex \+a used in the grepdiff1 test. + doesn't have special meaning in POSIX regular expressions if it's at the start of the expression. So the POSIX way to write that regular expression would be +a.

However, this is actually what the test used to do, until it was changed in https://github.com/twaugh/patchutils/commit/a6538753a51db973a05c9034ed78f2dd946453db with no explanation given. What was the motivation for that change?