fix(git): fix empty output when branch name contains '/' in git diff

[binsee]

Summary

• rtk git diff feature/user-auth produced silent empty output (exit 0) because
  normalize_diff_args treated any arg containing / as a file path and injected
  -- before it, causing git to interpret the branch name as a pathspec with no match.
• Replace the string heuristic looks_like_path() with a three-tier detection strategy:
  1. Explicit path prefixes (., ~) → always a path
  2. Contains path separator (/, \) → filesystem existence check to distinguish
     branch names (feature/auth, not on disk) from real paths (src/main.rs)
  3. Bare word with no separator → never inject -- (avoids misfiring when a file
     shares a name with a branch/ref, e.g. a file named main)
• Extract normalize_diff_args_impl with injectable path-checker for testability;
  update all existing tests to use mock existence checks; add three regression tests.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test — 1609 passed
• Manual: verified git diff HEAD~ src/deleted.rs (deleted file with /) still
  works correctly without -- injection
• Fixes: rtk git diff produces empty output when branch name contains / #1431

---

Investigated, implemented, and tested with the assistance of Claude Code (claude-sonnet-4-6).

[CLAassistant]

All committers have signed the CLA.

[pszymkowiak]

Real-binary test results ✅

Tested with binary built from this branch ():

# Test 1: branch name with / — was empty before fix
$ rtk git diff feature/user-auth
file.txt | 1 -
 1 file changed, 1 deletion(-)
--- Changes ---
file.txt
  @@ -1,2 +1 @@
  -auth feature change

# Test 2: real file path still works
$ rtk git diff file.txt
file.txt | 1 +
 1 file changed, 1 insertion(+)
--- Changes ---

# Test 3: 3-dot branch syntax feature/user-auth
$ rtk git diff HEAD...feature/user-auth  → works correctly

All cases pass. The filesystem-existence check correctly distinguishes feature/user-auth (not on disk → treat as ref) from file.txt (on disk → inject --).

[pszymkowiak]

Tested with real binary built from this branch. All cases confirmed working:

• rtk git diff feature/user-auth now shows diff (was silent empty before)
• rtk git diff file.txt still works (real path detection intact)
• rtk git diff HEAD...feature/user-auth (3-dot syntax) works
• 1609 tests pass, no regressions

[aeppling]

Hey, checking if path exist to differentiate branch / path is correct
Thanks for contributing