filter-branch: stop depending on Perl

filter-branch: stop depending on Perl

While git-filter-branch(1) is written as a shell script, the
`--state-branch` feature depends on Perl to save and extract the object
ID mappings. This can lead to subtle breakage though:

  - We execute `perl` directly without respecting the `PERL_PATH`
    configured by the distribution. As such, it may happen that we use
    the wrong version of Perl.

  - We install the script unchanged even if Perl isn't available at all
    on the system, so using `--state-branch` would lead to failure
    altogether in that case.

Fix this by dropping Perl and instead implementing the feature with
shell scripting exclusively.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>