The code in "blame_git.c" was mostly imported from git.git with only
minor changes. One of these changes was to use our own allocators
instead of git's `xmalloc`, but there's a subtle difference: `xmalloc`
would abort the program if unable to allocate any memory, bit
`git__malloc` doesn't. As we didn't check for memory allocation errors
in some places, we might inadvertently dereference a `NULL` pointer in
out-of-memory situations.

Convert multiple functions to return proper error codes and add calls to
`GIT_ERROR_CHECK_ALLOC` to fix this.