When trying to inflate a buffer, a GIT_ERROR was returned when malloc()
failed.

Fix this to return GIT_ENOMEM.

git__delta_apply used to return with a generic GIT_ERROR when allocating
memory for the delta failed.

Fix this to return GIT_ENOMEM.

"git_config_backend" have been renamed to "git_config_file", which
implements a generic interface to access a configuration file -- be it
either on disk, from a DB or whatever mumbojumbo.

I think this makes more sense.

We have a lot of utility methods that make path building trivial. Use
them!

A couple of config improvements

Fix a few minor typos in comments and error messages

Regarding "initialize" vs. "initialise", www.dict.cc says the first is American
English whereas the latter in British English. For consistency, we should
stick to American English.

index: Add API for unmerged entries

Move odb.c to the new error handling

test_lib: add return value to git_test