Taylor Blau wrote:

There are many functions in commit.h that are more related to shallow
repositories than they are to any sort of generic commit machinery.
Likely this began when there were only a few shallow-related functions,
and commit.h seemed a reasonable enough place to put them.

But, now there are a good number of shallow-related functions, and
placing them all in 'commit.h' doesn't make sense.

Sure.  For me, there are a few additional sources of motivation:

- shallow clone is a bit of a thorny feature, so I like having the
  indication of which source files are interacting with it

- this will give us a good place to put any overview documentation on
  the shallow API

This patch extracts a 'shallow.h', which takes all of the headers from
'commit.h' for functions which already exist in 'shallow.c'. We will
bring the remaining shallow-related functions defined in 'commit.c' in a
subsequent patch.

For now, move only the ones that already are implemented in 'shallow.c',
and update the necessary includes.

It's probably worth a mention of the builtin.h part here.

(By the way, I wouldn't be against propagating that to the callers,
to better match what https://include-what-you-use.org/ would enforce.)

Thanks,
Jonathan