split_commit_in_progress(): simplify & fix memory leak

split_commit_in_progress(): simplify & fix memory leak

This function did a whole lot of unnecessary work, such as reading in
four files just to figure out that, oh, hey, we do not need to look at
them after all because the HEAD is not detached.

Simplify the entire function to return early when possible, to read in
the files only when necessary, and to release the allocated memory
always (there was a leak, reported via Coverity, where we failed to
release the allocated strings if the HEAD is not detached).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>