ref-filter.c: find disjoint pattern prefixes

Since cfe004a5a9 (ref-filter: limit traversal to prefix, 2017-05-22),
the ref-filter code has sought to limit the traversals to a prefix of
the given patterns.

That code stopped short of handling more than one pattern, because it
means invoking 'for_each_ref_in' multiple times. If we're not careful
about which patterns overlap, we will output the same refs multiple
times.

For instance, consider the set of patterns 'refs/heads/a/*',
'refs/heads/a/b/c', and 'refs/tags/v1.0.0'. If we naïvely ran:

  for_each_ref_in("refs/heads/a/*", ...);
  for_each_ref_in("refs/heads/a/b/c", ...);
  for_each_ref_in("refs/tags/v1.0.0", ...);

we would see 'refs/heads/a/b/c' (and everything underneath it) twice.

Instead, we want to partition the patterns into disjoint sets, where we
know that no ref will be matched by any two patterns in different sets.
In the above, these are:

  - {'refs/heads/a/*', 'refs/heads/a/b/c'}, and
  - {'refs/tags/v1.0.0'}

Given one of these disjoint sets, what is a suitable pattern to pass to
'for_each_ref_in'? One approach is to compute the longest common prefix
over all elements in that disjoint set, and let the caller cull out the
refs they didn't want. Computing the longest prefix means that in most
cases, we won't match too many things the caller would like to ignore.

The longest common prefixes of the above are:

  - {'refs/heads/a/*', 'refs/heads/a/b/c'} -> refs/heads/a/*
  - {'refs/tags/v1.0.0'}                   -> refs/tags/v1.0.0

We instead invoke:

  for_each_ref_in("refs/heads/a/*", ...);
  for_each_ref_in("refs/tags/v1.0.0", ...);

Which provides us with the refs we were looking for with a minimal
amount of extra cruft, but never a duplicate of the ref we asked for.

Implemented here is an algorithm which accomplishes the above, which
works as follows:

  1. Lexicographically sort the given list of patterns.

  2. Initialize 'prefix' to the empty string, where our goal is to
     build each element in the above set of longest common prefixes.

  3. Consider each pattern in the given set, and emit 'prefix' if it
     reaches the end of a pattern, or touches a wildcard character. The
     end of a string is treated as if it precedes a wildcard. (Note that
     there is some room for future work to detect that, e.g., 'a?b' and
     'abc' are disjoint).

  4. Otherwise, recurse on step (3) with the slice of the list
     corresponding to our current prefix (i.e., the subset of patterns
     that have our prefix as a literal string prefix.)

This algorithm is 'O(kn + n log(n))', where 'k' is max(len(pattern)) for
each pattern in the list, and 'n' is len(patterns).

By discovering this set of interesting patterns, we reduce the runtime
of multi-pattern 'git for-each-ref' (and other ref traversals) from
O(N) to O(n log(N)), where 'N' is the total number of packed references.

Running 'git for-each-ref refs/tags/a refs/tags/b' on a repository with
10,000,000 refs in 'refs/tags/huge-N', my best-of-five times drop from:

  real    0m5.805s
  user    0m5.188s
  sys     0m0.468s

to:

  real    0m0.001s
  user    0m0.000s
  sys     0m0.000s

On linux.git, the times to dig out two of the latest -rc tags drops from
0.002s to 0.001s, so the change on repositories with fewer tags is much
less noticeable.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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