Instead of allocating a brand new buffer for each error string we want
to store, we can use a per-thread buffer to store the error string and
re-use the underlying storage. We already use the buffer to format the
string, so this mostly makes that more direct.

Normalize submodule urls before looking at them

Allow adding a submodule through git_index_add_bypath

refdb: delete a ref's reflog upon deletion

filter: make sure to close the stream even on error

Make libgit2 work on Windows Vista again

Increment `git__n_inits` before doing `init_once`.

Fixes #3318.

(fixes issue #3316)

Signed-off-by: Sven Strickroth <email@cs-ware.de>