Use the 7 first characters of the 40-chars long SHAs for shorter/cleaner URLs.
The collision probability is extremely low (since all SHAs are further
"namespaced" under the corresponding PR). In case of a collision, the second PR
will not be deployed, in order to avoid overwriting the original build.
(This is a design decision to keep the implementation simple. It can be changed
later if necessary.)
The previous clean-up code for PR directories on the preview server assumed that
all directories were named after the PR number. With the changes introduced
in #17640 it is possible to have PR directories that do not follow that naming
convention (e.g. "non-public" directories).
This PR ensures that both public and non-public directories are removed when
cleaning up.
Georgios Kalpakas