subtree: t7900: update for having the default branch name be 'main'

Most of the tests had been converted to support
`GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main`, but `contrib/subtree/t/`
hadn't.

Convert it.  Most of the mentions of 'master' can just be replaced with
'HEAD'.

Signed-off-by: Luke Shumaker <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LukeShu