git-prompt: don't use shell arrays

Arrays only existed in the svn-upstream code, used to:
- Keep a list of svn remotes.
- Convert commit msg to array of words, extract the 2nd-to-last word.

Except bash/zsh, nearly all shells failed load on syntax errors here.

Now:
- The svn remotes are a list of newline-terminated values.
- The 2nd-to-last word is extracted using standard shell substrings.
- All shells can digest the svn-upstream code.

While using shell field splitting to extract the word is simple, and
doesn't even need non-standard code, e.g. set -- $(git log -1 ...),
it would have the same issues as the old array code: it depends on IFS
which we don't control, and it's subject to glob-expansion, e.g. if
the message happens to include * or **/* (as this commit message just
did), then the array could get huge. This was not great.

Now it uses standard shell substrings, and we know the exact delimiter
to expect, because it's the match from our grep just one line earlier.

The new word extraction code also fixes svn-upstream in zsh, because
previously it used arr[len-2], but because in zsh, unlike bash, array
subscripts are 1-based, it incorrectly extracted the 3rd-to-last word.
symptom: missing upstream status in a git-svn repo: u=, u+N-M, etc.

The breakage in zsh is surprising, because it was last touched by
  commit d0583da838 (prompt: fix show upstream with svn and zsh),
claiming to fix exactly that. However, it only mentions syntax fixes.
It's unclear if behavior was fixed too. But it was broken, now fixed.

Note LF=$'\n' and then using $LF instead of $'\n' few times.
A future commit will add fallback for shells without $'...', so this
would be the only line to touch instead of replacing every $'\n' .

Shells which could run the previous array code:
- bash

Shells which have arrays but were broken anyway:
- zsh: 1-based subscript
- ksh93: no "local" (the new code can't fix this part...)
- mksh, openbsd sh, pdksh: failed load on syntax error: "for ((...))".

More shells which Failed to load due to syntax error:
- dash, free/net bsd sh, busybox-ash, Schily Bourne shell, yash.

Signed-off-by: Avi Halachmi (:avih) <avihpit@yahoo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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