gitweb: Use git-diff-tree patch output for commitdiff

gitweb: Use git-diff-tree patch output for commitdiff

Get rid of git_diff_print invocation in git_commitdiff and therefore
external diff (/usr/bin/diff) invocation, and use only git-diff-tree
to generate patch.

git_commitdiff and git_commitdiff_plain are collapsed into one
subroutine git_commitdiff, with format (currently 'html' which is
default format corresponding to git_commitdiff, and 'plain'
corresponding to git_commitdiff_plain) specified in argument.

Separate patch (diff) pretty-printing into git_patchset_body.
It is used in git_commitdiff.

Separate patch (diff) line formatting from git_diff_print into
format_diff_line function. It is used in git_patchset_body.

While at it, add $hash parameter to git_difftree_body, according to
rule that inner functions should use parameter passing, and not global
variables.

CHANGES TO OUTPUT:
 * "commitdiff" now products patches with renaming and copying
   detection (git-diff-tree is invoked with -M and -C options).
   Empty patches (mode changes and pure renames and copying)
   are not written currently. Former version broke renaming and
   copying, and didn't notice mode changes, like this version.

 * "commitdiff" output is now divided into several div elements
   of class "log", "patchset" and "patch".

 * "commitdiff_plain" now only generates X-Git-Tag: line only if there
   is tag pointing to the current commit. Former version which wrote
   first tag following current commit was broken[*1*]; besides we are
   interested rather in tags _preceding_ the commit, and _heads_
   following the commit. X-Git-Url: now is current URL; former version
   tried[*2*] to output URL to HTML version of commitdiff.

 * "commitdiff_plain" is generated by git-diff-tree, and has therefore
   has git specific extensions to diff format: "git diff" header and
   optional extended header lines.

FOOTNOTES
[*1*] First it generated rev-list starting from HEAD even if hash_base
parameter was set, second it wasn't corrected according to changes
made in git_get_references (formerly read_info_ref) output, third even
for older version of read_info_ref output it didn't work for multiple
tags pointing to the current commit (rare).

[*2*] It wrote URL for commitdiff without hash_parent, which produces
diff to first parent and is not the same as current diff if it is diff
of merge commit to non-first parent.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>