Fix for pre-commit hook failure on bash 4

This PR fixes the issue described here.
awslabs/git-secrets#45 (comment)

Author: Sze Siong Teo

git-secrets

@@ -102,7 +102,7 @@ scan_history() {
   # Looks for differences matching the patterns, reduces the number of revisions to scan
   local to_scan=$(git log --all -G"${combined_patterns}" --pretty=%H)
   # Scan through revisions with findings to normalize output
-  output=$(git_grep "" "${to_scan}")
+  output=$(GREP_OPTIONS= LC_ALL=C git grep -nwHEI "${combined_patterns}" $to_scan)
   process_output $? "${output}"
 }
 
@@ -112,7 +112,7 @@ git_grep() {
   local options="$1"; shift
   local files=$@ combined_patterns=$(load_combined_patterns)
   [ -z "${combined_patterns}" ] && return 1
-  GREP_OPTIONS= LC_ALL=C git grep -nwHEI ${options} "${combined_patterns}" $files
+  GREP_OPTIONS= LC_ALL=C git grep -nwHEI ${options} "${combined_patterns}" -- $files
 }
 
 # Performs a regular grep, taking into account patterns and recursion.