contrib/subtree: Store subtree sources in .gitsubtree

Add the prefix, repository and refspec in the file .gitsubtree when
git subtree add is used. Then when a git subtree push or pull is needed
the repository and refspec from .gitsubtree are used as the default
values.

Having to remember what subtree came from what source is a waste of
developer memory and doesn't transfer easily to other developers.

git subtree push/pull operations would typically be to/from the same
source that the original subtree was cloned from with git subtree add.

The <repository> and <refspec>, or <commit>, used in the git subtree add
operation are stored in .gitsubtree. One line each, delimited by a space:
"<prefix> <repository> <refspec>" or "<prefix> . <commit>".

Subsequent git subtree push/pull operations now default to the values
stored in .gitsubtree, unless overridden from the command line.

The .gitsubtree file should be tracked as part of the repo as it
describes where parts of the tree came from and can be used to update
to and from that source.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>
contrib/subtree/t: Added tests for .gitsubtree support

add: ensure details are added to .gitsubtree
push: check for a SHA1 update line
pull: add a file on one subtree, push it to a branch, then pull into
    another subtree containing the same branch and confirm the files match
add: ensure stale .gitsubtree entry is replaced

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>
contrib/subtree: update documentation

Indicate that repository and refspec are now optional on push and pull.

Add notes to add, push and pull about storing values in .gitsubtree
and their use as default values.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>