Fix git diff call

[fthomas]

git diff fails if there is a branch and file with the same name:

2021-11-23 14:16:10,530 INFO  Create branch update/bloop-config-1.4.11
2021-11-23 14:16:18,210 ERROR Steward com-lihaoyi/mill failed
git diff --name-only main update/bloop-config-1.4.11' exited with code 128
fatal: ambiguous argument 'main': both revision and filename
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

This change therefore adds -- after the branches in our git diff call.

[codecov]

Codecov Report

Merging #2372 (54d2d84) into master (a944e32) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2372   +/-   ##
=======================================
  Coverage   80.46%   80.46%           
=======================================
  Files         139      139           
  Lines        2396     2396           
  Branches       51       51           
=======================================
  Hits         1928     1928           
  Misses        468      468           

Impacted Files	Coverage Δ
...n/scala/org/scalasteward/core/git/FileGitAlg.scala	98.33% <100.00%> (ø)

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