describe: Avoid unnecessary warning when using --all

describe: Avoid unnecessary warning when using --all

In 212945d4 ("Teach git-describe to verify annotated tag names
before output") git-describe learned how to output a warning if
an annotated tag object was matched but its internal name doesn't
match the local ref name.

However, "git describe --all" causes the local ref name to be
prefixed with "tags/", so we need to skip over this prefix before
comparing the local ref name with the name recorded inside of the
tag object.

Patch-by: René Scharfe <redacted>
Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>