Replace with submatch(backreference) is not working

[fujimura]

git-gsub is using regexp package for replacing string, which depends on re2 and re2 doesn't support submatch.

Replacing with submatch is undocumented(yet partially test) feature, but I hope this to be supported officially.

Failing test case can be seen in #22.

regexp package - regexp - pkg.go.dev Syntax · google/re2 Wiki