support/scripts/generate-gitlab-ci-yml: improve test-pkg support

Following the example of test-pkg config described in commit
"12c7a05d utils/test-pkg: add gitlab-ci support" to test a defconfig
fragment that contains a disabled option is currently possible, but
it do requires one to change the git config core.commentChart so the
lines starting with "#" are not discarded by git when creating/editing
the commit message.

For instance, without the indentation the 3rd line below would be
excluded from the commit message when the editor is closed:
    test-pkg config:
    SOME_OPTION=y
    # OTHER_OPTION is not set
    SOME_VARIABLE="some value"

Requiring to change git configs is not very nice.
So make the developer's life easier by changing the sed expression to
remove indentation with spaces from a defconfig fragment found on a
commit message.
For instance these lines become valid and generate a defconfig fragment
without the indentation of one space to be tested in GitLab CI:
test-pkg config:
 SOME_OPTION=y
 # OTHER_OPTION is not set
 SOME_VARIABLE="some value"

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>