Improve script performance by disabling the warning from git filter-branch

[Gems]

Relates to #32 .

Add FILTER_BRANCH_SQUELCH_WARNING=1 environment variable export to disable git filter-branch warning that affects script performance.

The goit filter-branch command warning adds delay for several seconds to each command execution making the redate command running quite slow.

WARNING: git-filter-branch has a glut of gotchas generating mangled history
	 rewrites.  Hit Ctrl-C before proceeding to abort, then use an
	 alternative filtering tool such as 'git filter-repo'
	 (https://github.com/newren/git-filter-repo/) instead.  See the
	 filter-branch manual page for more details; to squelch this warning,
	 set FILTER_BRANCH_SQUELCH_WARNING=1.
Proceeding with filter-branch...