diff: add --word-diff option that generalizes --color-words

This teaches the --color-words engine a more general interface that
supports two new modes:

* --word-diff=plain, inspired by the 'wdiff' utility (most similar to
  'wdiff -n <old> <new>'): uses delimiters [-removed-] and {+added+}

* --word-diff=porcelain, which generates an ad-hoc machine readable
  format:
  - each diff unit is prefixed by [-+ ] and terminated by newline as
    in unified diff
  - newlines in the input are output as a line consisting only of a
    tilde '~'

Both of these formats still support color if it is enabled, using it
to highlight the differences.  --color-words becomes a synonym for
--word-diff=color, which is the color-only format.  Also adds some
compatibility/convenience options.

Thanks to Junio C Hamano and Miles Bader for good ideas.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 37 insertions, 5 deletions

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 60e922e6ef..a616ca589f 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -126,11 +126,39 @@ any of those replacements occurred.
 	gives the default to color output.
 	Same as `--color=never`.
 
---color-words[=<regex>]::
-	Show colored word diff, i.e., color words which have changed.
-	By default, words are separated by whitespace.
+--word-diff[=<mode>]::
+	Show a word diff, using the <mode> to delimit changed words.
+	By default, words are delimited by whitespace; see
+	`--word-diff-regex` below.  The <mode> defaults to 'plain', and
+	must be one of:
++
+--
+color::
+	Highlight changed words using only colors.  Implies `--color`.
+plain::
+	Show words as `[-removed-]` and `{+added+}`.  Makes no
+	attempts to escape the delimiters if they appear in the input,
+	so the output may be ambiguous.
+porcelain::
+	Use a special line-based format intended for script
+	consumption.  Added/removed/unchanged runs are printed in the
+	usual unified diff format, starting with a `+`/`-`/` `
+	character at the beginning of the line and extending to the
+	end of the line.  Newlines in the input are represented by a
+	tilde `~` on a line of its own.
+none::
+	Disable word diff again.
+--
++
+Note that despite the name of the first mode, color is used to
+highlight the changed parts in all modes if enabled.
+
+--word-diff-regex=<regex>::
+	Use <regex> to decide what a word is, instead of considering
+	runs of non-whitespace to be a word.  Also implies
+	`--word-diff` unless it was already enabled.
 +
-When a <regex> is specified, every non-overlapping match of the
+Every non-overlapping match of the
 <regex> is considered a word.  Anything between these matches is
 considered whitespace and ignored(!) for the purposes of finding
 differences.  You may want to append `|[^[:space:]]` to your regular
@@ -142,6 +170,10 @@ The regex can also be set via a diff driver or configuration option, see
 linkgit:gitattributes[1] or linkgit:git-config[1].  Giving it explicitly
 overrides any diff driver or configuration setting.  Diff drivers
 override configuration settings.
+
+--color-words[=<regex>]::
+	Equivalent to `--word-diff=color` plus (if a regex was
+	specified) `--word-diff-regex=<regex>`.
 endif::git-format-patch[]
 
 --no-renames::
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index d892e642ed..7554fcd07f 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -360,7 +360,7 @@ patterns are available:
 Customizing word diff
 ^^^^^^^^^^^^^^^^^^^^^
 
-You can customize the rules that `git diff --color-words` uses to
+You can customize the rules that `git diff --word-diff` uses to
 split words in a line, by specifying an appropriate regular expression
 in the "diff.*.wordRegex" configuration variable.  For example, in TeX
 a backslash followed by a sequence of letters forms a command, but