sequencer: pass absolute GIT_DIR to exec commands

sequencer: pass absolute GIT_DIR to exec commands

When we replaced the old shell script based interactive rebase in
commmit 18633e1a22a6 ("rebase -i: use the rebase--helper builtin",
2017-02-09) we introduced a regression of functionality in that the
GIT_DIR would be sent to the environment of the exec command as-is.

This generally meant that it would be passed as "GIT_DIR=.git", which
causes problems for any exec command that wants to run git commands in
a subdirectory.

This isn't a very large regression, since it is not that likely that the
exec command will run a git command, and even less likely that it will
need to do so in a subdir. This regression was discovered by a build
system which uses git-describe to find the current version of the build
system, and happened to do so from the src/ sub directory of the
project.

Fix this by passing in the absolute path of the git directory into the
child environment.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>