fix: do not allow GREP_XXX env vars from breaking `rlocation` (#36)

The grep command can be influenced by `GREP_XXX` environment variables.
If set, they can affect the behavior/output of the command, causing
`rlocation` to fail.

shell/runfiles/runfiles.bash

diff --git a/shell/runfiles/runfiles.bash b/shell/runfiles/runfiles.bash
index f618b2c..e2dfd20 100644
--- a/shell/runfiles/runfiles.bash
+++ b/shell/runfiles/runfiles.bash
@@ -113,7 +113,9 @@
 # Does not exit with a non-zero exit code if no match is found and performs a case-insensitive
 # search on Windows.
 function __runfiles_maybe_grep() {
-  grep $_RLOCATION_GREP_CASE_INSENSITIVE_ARGS "$@" || test $? = 1;
+  # The GREP_XXX variables influence how grep behaves. Specifically, they can
+  # affect the output from the grep command.
+  GREP_COLOR="" GREP_OPTIONS="" grep $_RLOCATION_GREP_CASE_INSENSITIVE_ARGS "$@" || test $? = 1;
 }
 export -f __runfiles_maybe_grep
 

tests/runfiles/runfiles_test.bash

diff --git a/tests/runfiles/runfiles_test.bash b/tests/runfiles/runfiles_test.bash
index fb61b86..f738b3d 100755
--- a/tests/runfiles/runfiles_test.bash
+++ b/tests/runfiles/runfiles_test.bash
@@ -522,6 +522,13 @@
   [[ -z "${RUNFILES_MANIFEST_FILE:-}" ]] || fail
 }
 
+function test_with_grep_env_vars_set() {
+  # These influence how grep behaves.
+  export GREP_COLOR='1;35;40'
+  export GREP_OPTIONS='--color=always'
+  test_init_manifest_based_runfiles
+}
+
 function main() {
   local -r manifest_file="${RUNFILES_MANIFEST_FILE:-}"
   local -r dir="${RUNFILES_DIR:-}"