apply: make parse_git_diff_header public

apply: make parse_git_diff_header public

Make 'parse_git_header()' (renamed to 'parse_git_diff_header()') a
"public" function in apply.h, so we can re-use it in range-diff in a
subsequent commit.  We're renaming the function to make it clearer in
other parts of the codebase that we're talking about a diff header and
not just any header.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>