The `git_error_set` function is useful for callers who implement
backends and advanced callbacks.  Expose it.

Update the version number in main to v1.5.0-alpha.  This helps people
understand that the main builds are not part of the v1.4.0 release
train.

We use "alpha" to indicate builds out of main (or nightlies) as semver
v2 requires the prerelease component is compared lexicographically.
Thus, our "beta" and "rc" releases should follow.

merge: fix overlap between GIT_MERGE_FILE_FAVOR__CONFLICTED and GIT_MERGE_FILE_SIMPLIFY_ALNUM

We made the flags that enable recursive merge internal, on the
assumption that nobody would want them and they're hard to reason about.
(Giving people an option that nobody wants is just extra noise.)

However, it made it hard for _us_ to reason about.  There's no good
reason to keep it private, let's just make it public and push that
cognitive load onto our poor users.  But they should expect it, they're
dealing with git, after all.

When we know the file size (because we're producing it from a working
directory iterator, or an index with an up-to-date cache) then set a
flag indicating as such.  This removes the ambiguity about a 0 file
size, which could indicate that a file exists and is 0 bytes, or that we
haven't read it yet.

Fetch by object id

Allocate flags in git_merge_flag_t and git_merge_file_flag_t
for internal usage to prevent accidental double allocation.