When the encoding header changed to be treated as an additional
header, the EOL pointer started to point to the byte after the LF,
making the git__strndup call copy the LF into the value.

Increase the EOL pointer value after copying the data to keep the rest
of the semantics but avoid copying LF.

This moves the check for the "encoding" header into a loop which
is just scanning for non-required headers at the end of a commit
header.  That loop will skip unrecognized lines (including header
continuation lines) until a terminating completely blank line is
found, and only then does it move to reading the commit message.

Conflicts:
	src/branch.c
	tests-clar/refs/branches/create.c