merging-rebase: set both GIT_SEQUENCE_EDITOR and GIT_EDITOR to modify the insn-cheet

Somehow git rebase -i seems to ignore GIT_EDITOR and only uses
GIT_SEQUENCE_EDITOR. Set both variables to not introduce a regression.

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
Merge pull request #129 from snaewe/fix-merging-rebase

merging-rebase: set both GIT_SEQUENCE_EDITOR and GIT_EDITOR to modify the insn-cheet