Do not rely on IFS from env when loading combined patterns

[michaelfindlater]

What?

• load patterns delimited by '\n' rather than default IFS ' \t\n' (space, tab, newline)
• use while instead of for to avoid modifying/relying on the global value of IFS (general best practice)
• PR also opened on the awslabs/git-secrets side

Why?

• combined_patterns() concatenates patterns with | for use in a single grep command
• using the default value of IFS means patterns containing a space character delimited by space, rather than newline
• this causes unexpected behavior (false matches from words split by space)

Expected

For example, consider the following list of patterns:

^my-regex$
^my regex with spaces$

load_combined_patterns() should transform them into: ^my-regex$|^my regex with spaces$

Actual

The above example ends up as ^my-regex$|^my|regex|with|spaces$.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[tomoyat1]

LGTM 👍