Avoid unnecessary strlen() calls

Avoid unnecessary strlen() calls

This is a micro-optimization that grew out of the mailing list discussion
about "strlen()" showing up in profiles.

We used to pass regular C strings around to the low-level tree walking
routines, and while this worked fine, it meant that we needed to call
strlen() on strings that the caller always actually knew the size of
anyway.

So pass the length of the string down wih the string, and avoid
unnecessary calls to strlen(). Also, when extracting a pathname from a
tree entry, use "tree_entry_len()" instead of strlen(), since the length
of the pathname is directly calculable from the decoded tree entry itself
without having to actually do another strlen().

This shaves off another ~5-10% from some loads that are very tree
intensive (notably doing commit filtering by a pathspec).

Signed-off-by: Linus Torvalds  <torvalds@linux-foundation.org>"
Signed-off-by: Junio C Hamano <junkio@cox.net>