use 'tree-ish' instead of 'treeish'

use 'tree-ish' instead of 'treeish'

Replace 'treeish' in documentation and comments with 'tree-ish' to
match gitglossary(7).

The only remaining instances of 'treeish' are:
  * variable, function, and macro names
  * "(also treeish)" in the definition of tree-ish in gitglossary(7)

Signed-off-by: Richard Hansen <redacted>
Signed-off-by: Junio C Hamano <redacted>