ci_main: fix cred helper handle 'store'

[sluongng]

In a previous change, we managed to detect when the credential helper
was invoked by git with the get operation and provide a special
espcape hatch for it.

However, get is not the only operation that git will call us with.
The other operations include store and erase in the current
git-credentials spec.

Adjust the credential helper detection when parsing flags to make sure
we don't run into infinite loop with store and erase operations
(or future new operations).