Fix CaseConflicts handling to work with larger file sets

When running `overcommit --run` with a large set of files,
`CaseConflicts` would call `git ls-tree` with an argument list too large
for the OS to handle. This was exacerbated by the fact that we were
passing the same directory multiple times since we were not returning
the unique set of directories.

Author: sds

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Fix pre-commit hook bug where amending a commit which breaks a symlink would
   result in that symlink not being included in the list of modified files
+* Fix `CaseConflicts` pre-commit hook handling of large sets of files
 
 ## 0.26.0
 

lib/overcommit/hook/pre_commit/case_conflicts.rb

@@ -6,7 +6,7 @@ def run
       repo_files = Set.new(applicable_files)
 
       unless Overcommit::GitRepo.initial_commit?
-        paths = repo_files.map { |file| File.dirname(file) + File::SEPARATOR }
+        paths = repo_files.map { |file| File.dirname(file) + File::SEPARATOR }.uniq
         repo_files += Overcommit::GitRepo.list_files(paths)
       end