Add git-reup and friends

docs/src/git-upstream-diff.txt

Index: docs/src/git-upstream-diff.txt
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+git-upstream-diff(1)
+====================
+
+NAME
+----
+git-upstream-diff -
+include::_git-upstream-diff_desc.helper.txt[]
+
+SYNOPSIS
+--------
+[verse]
+'git upstream-diff' [--wordwise] [<extra args for git-diff>*]
+
+DESCRIPTION
+-----------
+
+Shows a diff beween your current branch and it's upstream. This is 'roughly' the
+same as:
+
+----
+git diff --patience -C -C HEAD@{upstream}  <1> <2>
+----
+<1> `-C -C` detects file copies/renames
+<2> `--patience` uses the patience-diff algorithm, which tends to produce nicer
+  diffs in many cases.
+
+The difference is that `HEAD@{upstream}` is actually the tagged merge base of
+your branch (See linkgit:git-rebase-update[1]). This means that if your upstream
+branch was rebased, but you haven't yet rebased the current branch on top of it,
+you'll still see an accurate diff compared to just diffing against
+`@{upstream}`.
+
+The `--wordwise` option also allows `git-diff` to do word-by-word comparison
+in a semi-intelligent way. However, sometimes it can produce surprising results,
+so it is disabled by default.
+
+
+OPTIONS
+-------
+
+--wordwise::
+  Print a colorized word-wise diff instead of a line-wise diff.
+
+<extra args for git-diff>::
+  Extra arguments are included in the invocation of linkgit:git-diff[1]. These
+  can be anything that `git-diff` normally takes.
++
+--
+`--stat`;;
+  This is particularly useful to show 'which' files have been changed in
+  comparison to the upstream branch.
+`-- <filename patterns>*`;;
+  Restrict the diff to only show the diff for given files compared to the
+  upstream.
+--
+
+
+CONFIGURATION VARIABLES
+-----------------------
+
+depot-tools.upstream-diff.default-args
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A list-configuration variable. Each instance of this config variable will be
+prepended to all invocations of `git upstream-diff`, as if you had passed them
+on the command line.
+
+include::_aliases.txt[]
+
+----
+[alias]
+  git udiff = upstream-diff
+----
+
+SEE ALSO
+--------
+linkgit:git-rebase-update[1]
+
+include::_footer.txt[]
+
+// vim: ft=asciidoc: