am: don't persist keepcr flag

am: don't persist keepcr flag

The keepcr flag is only used in the split_patches function, which is
only called before a patch application has to stopped for user input,
not after resuming. It is therefore unnecessary to persist the
flag. This seems to have been the case since it was introduced in
ad2c928 (git-am: Add command line parameter `--keep-cr` passing it to
git-mailsplit, 2010-02-27).

Signed-off-by: Martin von Zweigbergk <redacted>
Signed-off-by: Junio C Hamano <redacted>