fast-export: do not modify memory from get_commit_buffer

fast-export: do not modify memory from get_commit_buffer

fast-export's helper function find_encoding() takes a `const char *`, but
modifies that memory despite the `const`.  Ultimately, this memory came
from get_commit_buffer(), and you're not supposed to modify the memory
that you get from get_commit_buffer().

So, get rid of find_encoding() in favor of commit.h:find_commit_header(),
which gives back a string length, rather than mutating the memory to
insert a '\0' terminator.

Because find_commit_header() detects the "\n\n" string that separates the
headers and the commit message, move the call to be above the
`message = strstr(..., "\n\n")` call.  This helps readability, and allows
for the value of `encoding` to be used for a better value of "..." so that
the same memory doesn't need to be checked twice.  Introduce a
`commit_buffer_cursor` variable to avoid writing an awkward
`encoding ? encoding + encoding_len : committer_end` expression.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>