Makefile: keep LIB_H entries together and sorted

As a follow-up to 60d24dd25 (Makefile: fold XDIFF_H and VCSSVN_H into
LIB_H), let the unconditional additions to LIB_H form a single sorted
list.  Also drop the duplicate entry for xdiff/xdiff.h, which was easy
to spot after sorting.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
sha1_name: pass object name length to diagnose_invalid_sha1_path()

The only caller of diagnose_invalid_sha1_path() extracts a substring from
an object name by creating a NUL-terminated copy of the interesting part.
Add a length parameter to the function and thus avoid the need for an
allocation, thereby simplifying the code.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
rev-parse: clarify documentation of $name@{upstream} syntax

"git rev-parse" interprets string in string@{upstream} as a name of
a branch not a ref. For example, refs/heads/master@{upstream} looks
for an upstream branch that is merged by git-pull to ref
refs/heads/refs/heads/master not to refs/heads/master.

However the documentation could mislead a user to believe that the
string is interpreted as ref.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>