fix(@angular/cli): add blank line in migration commit message

[drumonii]

When using the -C option of update, the commit message of the migration is missing a blank line between the subject and body.

For example,

Previously was missing blank line after the commit subject:

@angular/cli migration - rename-browserslist-config
Update Browserslist configuration file name to '.browserslistrc' from deprecated 'browserslist'.

Now fixed:

@angular/cli migration - rename-browserslist-config

Update Browserslist configuration file name to '.browserslistrc' from deprecated 'browserslist'.

[alan-agius4]

Thanks, LGTM

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