Fixes several issues that could occur when running subtree split on large
repos with more complex history.

 1. A merge commit could bypass the known start point of the subtree, which
    would cause the entire history to be processed recursively, leading to a
    stack overflow / segfault after reading a few hundred commits. Older
    commits are now explicitly recorded as irrelevant so that the recursive
    process can terminate on any mainline commit rather than only on subtree
    joins and initial commits.
    
    
 2. It is possible for a repo to contain subtrees that lack the metadata
    that is usually present in add/join commit messages (git-svn at least
    can produce such a structure). The new use/ignore/map commands allow the
    user to provide that information for any problematic commits.
    
    
 3. A mainline commit that does not contain the subtree folder could be
    erroneously identified as a subtree commit, which would add the entire
    mainline history to the subtree. Commits will now only be used as is if
    all their parents are already identified as subtree commits. While the
    new code can still be tripped up by unusual folder structures, the
    completely unambiguous solution turned out to involve a significant
    performance penalty, and the new ignore / use commands provide a
    workaround for that scenario.

Tom Clarkson (7):
  subtree: handle multiple parents passed to cache_miss
  subtree: exclude commits predating add from recursive processing
  subtree: persist cache between split runs
  subtree: add git subtree map command
  subtree: add git subtree use and ignore commands
  subtree: more robustly distinguish subtree and mainline commits
  subtree: document new subtree commands

 contrib/subtree/git-subtree.sh  | 185 ++++++++++++++++++++++++++------
 contrib/subtree/git-subtree.txt |  24 +++++
 2 files changed, 177 insertions(+), 32 deletions(-)


base-commit: af6b65d45ef179ed52087e80cb089f6b2349f4ec
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-493%2Ftqc%2Ftqc%2Fsubtree-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-493/tqc/tqc/subtree-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/493
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