uniquePrefixMatch is exported and covered for unique, shared, last,
exact, absent, and empty cases, and a fixture repository with two commits
whose names share a seven-digit prefix shows that the shared prefix is
accepted while a longer unique prefix is rejected.
Refs #5852
verify-repository-references asked git cat-file for each of about 26,000
abbreviated hexadecimal candidates, which took 135 s on a network
filesystem. One --batch-all-objects listing (about 1.2 million objects,
0.6 s) now feeds an in-memory prefix search; a candidate still counts
only when its prefix names exactly one available object and that object
is a commit. The gate drops from 166.8 s to 15.7 s, and both
implementations classify 27,050 real and synthetic candidates
identically.
Refs #5852
Yichen Jiang