We now have `_fromstate` and `_fromstate_on_head`, the latter of which updates
the current branch to point to the new commit.

Currently git_commit_create() takes on creating a commit and updating a
reference. Provide a better interface by splitting up each of the
concerns into named functions for each.

git_commit_create() will only create the commit, it will not modify any
reference. git_commit_create_on() takes a reference name to update and
git_commit_create_on_head() is a convenience function to update HEAD.

This variant of the commit creation function takes the reference to
update, the tree and the parents from the current branch, index and
merging state, allowing for simpler use of a common use-case.

pathspec: improve git_pathspec_flag_t doc rendering

config: specify how we match the regular expressions

By placing docs per enum value rather than in a large block,
the automated doc generation tool can make nicer docs,
as could other automated tools, such as
the mooted https://github.com/libgit2/git2go/issues/427.

The current rendering is somewhat ugly:

https://libgit2.github.com/libgit2/#HEAD/type/git_pathspec_flag_t

No textual changes, just reorganization.