cat-file: fix mailmap application for different author and committer

The git cat-file command with --mailmap option fails to apply mailmap
transformations to the committer field when the author and committer
identities are different. This occurs due to a missing newline handling
in apply_mailmap_to_header() after processing each identity line.

When rewrite_ident_line() processes an identity, it stops at the end
of the identity data (e.g., "Author Name <email> timestamp"), but
doesn't account for the trailing newline. The current code adds the
identity length to buf_offset but fails to advance past the newline
character. This causes the next iteration to start parsing from the
newline instead of the beginning of the next header line, making it
impossible to match subsequent headers like "committer".

Additionally, rewrite_ident_line() may reallocate the buffer during
its operation. Any code using pointers into the old buffer would be
using invalid memory after such a reallocation.

This bug was introduced in e9c1b0e3 (revision: improve
commit_rewrite_person(), 2022-07-19) when the much simpler version of
commit_rewrite_person() that worked on one "person header" at a time
was rewritten to use the current apply_mailmap_to_header() function.
The original implementation processed author and committer separately,
but the rewrite introduced this loop-based approach that failed to
properly handle the transition between identity lines.

Let's fix this by addressing both issues:
1. After processing an identity line, we now check if we're at a
newline and advance past it, ensuring the next header line is
parsed correctly.
2. We recompute the buffer position after rewrite_ident_line() to
handle potential buffer reallocation.

This ensures that all identity headers in commit and tag objects are
consistently processed regardless of whether the author and committer
are the same person.

Reported-by: Vasilii Iakliushin <viakliushin@gitlab.com>
Reviewed-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>