t7401: use 'short' instead of 'verify' and cut in rev-parse calls

t7401: use 'short' instead of 'verify' and cut in rev-parse calls

'git rev-parse' can limit the number of characters in the hash it
outputs using the '--short' option, thereby, making the 'cut' invocation
redundant. Since using '--short' implies '--verify' as well, we can
safely replace the latter with the former. This change results in the
helper functions getting the hash in the same way 'summary' gets the
hash internally.

So, avoid the unnecessary invocation to 'cut' in the helper
functions.

Mentored-by: Christian Couder <redacted>
Mentored-by: Kaartic Sivaraam <redacted>
Signed-off-by: Shourya Shukla <redacted>
Signed-off-by: Junio C Hamano <redacted>