gpg-interface: fix strip_cr_before_lf to only remove CR before LF

gpg-interface: fix strip_cr_before_lf to only remove CR before LF

c4adea82c5 (Convert CR/LF to LF in tag signatures, 2008-07-11)
introduced CR stripping for GPG output on Windows, but intentionally
stripped all CR characters unconditionally to "keep the code
simpler", even though only CRLF sequences (Windows line endings)
needed to be normalized.

Later 2f47eae2a1 (Split GPG interface into its own helper library,
2011-09-07) moved the code into gpg-interface.c, and 29b315778e (ssh
signing: add ssh key format and signing code, 2021-09-10) extracted
it into the remove_cr_after() helper when adding SSH signing
support, while noticing that it unconditionally strips all CRs,
leaving a NEEDSWORK comment.

Fix the loop to skip CR only when immediately followed by LF,
keeping lone trailing CR characters intact.  Rename the function to
strip_cr_before_lf to reflect its corrected behavior, and update
both call sites and their comments accordingly.

Signed-off-by: Antonio De Stefani <redacted>
Signed-off-by: Junio C Hamano <redacted>