strbuf: introduce strbuf_strip_file_from_path()

strbuf: introduce strbuf_strip_file_from_path()

The strbuf_parent_directory() method was added as a static method in
contrib/scalar by d0feac4e8c0 (scalar: 'register' sets recommended
config and starts maintenance, 2021-12-03) and then removed in
65f6a9eb0b9 (scalar: constrain enlistment search, 2022-08-18), but now
there is a need for a similar method in the bundle URI feature.

Re-add the method, this time in strbuf.c, but with a new name:
strbuf_strip_file_from_path(). The method requirements are slightly
modified to allow a trailing slash, in which case nothing is done, which
makes the name change valuable.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>