git psykorebase --continue is broken on macOS

Problem

psykorebasing a branch on another, and having a resolved conflict:

$ git psykorebase --continue
Couldn't continue rebasing on ...

Analysis

psykorebase depends on sed, however the macOS version does not have the feature needed.

psykorebase has this line under the --continue flag:

    set $(echo "$TARGET_BRANCH" | sed -e "s/-rebased-on-top-of-/\n/g")

which is supposed to be splitting the branch name into two lines, to be later used as branch names.

macOS sed has a problem with the \n substitution, as it expects the literal newline character after \ not n, so the above code substitutes n instead of newline.

Relevant macOS documentation

$ man sed
SED(1)                    BSD General Commands Manual                   SED(1)

NAME
     sed -- stream editor
...
     [2addr]s/regular expression/replacement/flags
...
             A line can be split by substituting a newline character into it.  To specify a newline character in the replacement string, precede it with a back-slash.

sed compatibility comparison/guide

https://riptutorial.com/sed/topic/9436/bsd-macos-sed-vs--gnu-sed-vs--the-posix-sed-specification

In replacement strings used with the s command, assume that NO control-character escape sequences are supported, so, again, include control chars. as literals, as above.

Linux only:
sed 's/-/\t/' <<<$'a-b' # -> 'a<tab>b'
macOS and Linux:
sed 's/-/'$'\t''/' <<<'a-b'
sed 's/-/'"$(printf '\t')"'/' <<<'a-b'

In this case the working solution is a bit more complex (tested only on macOS):

    set $(echo "$TARGET_BRANCH" | sed -e "$(printf 's/-rebased-on-top-of-/\\\n/g')")

Start in the psykorebase implementation at the --continue flag and inspect how TARGET_BRANCH is split with sed. Reproduce the failure on macOS, then verify that a resolved conflict can continue successfully without breaking the existing Linux behavior.