ident: rename commit_rewrite_person() to apply_mailmap_to_header()

ident: rename commit_rewrite_person() to apply_mailmap_to_header()

commit_rewrite_person() takes a commit buffer and replaces the idents
in the header with their canonical versions using the mailmap mechanism.
The name "commit_rewrite_person()" is misleading as it doesn't convey
what kind of rewrite are we going to do to the buffer. It also doesn't
clearly mention that the function will limit itself to the header part
of the buffer. The new name, "apply_mailmap_to_header()", expresses the
functionality of the function pretty clearly.

We intend to use apply_mailmap_to_header() in git-cat-file to replace
idents in the headers of commit and tag object buffers. So, we will be
extending this function to take tag objects buffer as well and replace
idents on the tagger header using the mailmap mechanism.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>