test(project): copy one template repository instead of building each with four git calls

[omartelo]

internal/project is the slowest package on the Windows runner (82s in the #807 run, 6s on Linux) and almost none of it is a slow test: the suite spawns git 1,431 times, about 57ms a process on Windows. Half of those only build fixtures, and initRepo, called by ~95 tests, ran four of them (init, config, add, commit) to reach the same repository every time.

• initRepo builds that repository once per package run and hands each test an os.CopyFS copy plus one git update-index --refresh. The refresh is required: the copied index carries the template's inodes and ctimes, so the first git read would rewrite it, which TestSnapshotTreeLeavesTheRepositoryAlone catches.
• Skip semantics are kept: a failed git init skips (no usable git), a failure after it fails the test. TestMain removes the template.
• The author/committer env moves to testGitIdentity, shared by gitIn and the template build.

No assertion changes. Locally: git calls 1,431 → 1,036, package green at -count=5, go vet clean for linux, windows and darwin. The Windows number comes from this PR's Timings table.