Consider
$ complete -c foo -a 'aab aaB' -f
$ foo A<TAB>
since 28d67c8 we would insert the common prefix AND show the pager.
Due to case-insensitive comparison, "b/B" was considered to be part
of the prefix. Since the prefix is added to each pager item [1]
we get wrong results. Fix this by removing the insensitive comparison
between completions - I don't think it was of much use anyway.
Commandline tokens are still matched case-insensitively, this is
just about completions.
Test this by running interactive fish inside tmux (pexpect's terminal
emulation not have enough capabilities). Also add tests for recent
interactive regressions #7526 and #7738.
Closes#3978
[1]: b38a23a would solve this differently by giving every pager item
its own prefix, but was reverted since it needs more fixes.
