doc: generate a list of valid merge tools

Use the show_tool_names() function to build lists of all
the built-in tools supported by difftool and mergetool.
This frees us from needing to update the documentation
whenever a new tool is added.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/.gitignore

@@ -9,4 +9,5 @@ gitman.info
 howto-index.txt
 doc.dep
 cmds-*.txt
+mergetools-*.txt
 manpage-base-url.xsl

Documentation/Makefile

@@ -202,7 +202,11 @@ install-html: html
 #
 # Determine "include::" file references in asciidoc files.
 #
-doc.dep : $(wildcard *.txt) build-docdep.perl
+docdep_prereqs = \
+	mergetools-list.made $(mergetools_txt) \
+	cmd-list.made $(cmds_txt)
+
+doc.dep : $(docdep_prereqs) $(wildcard *.txt) build-docdep.perl
 	$(QUIET_GEN)$(RM) $@+ $@ && \
 	$(PERL_PATH) ./build-docdep.perl >$@+ $(QUIET_STDERR) && \
 	mv $@+ $@
@@ -226,13 +230,27 @@ cmd-list.made: cmd-list.perl ../command-list.txt $(MAN1_TXT)
 	$(PERL_PATH) ./cmd-list.perl ../command-list.txt $(QUIET_STDERR) && \
 	date >$@
 
+mergetools_txt = mergetools-diff.txt mergetools-merge.txt
+
+$(mergetools_txt): mergetools-list.made
+
+mergetools-list.made: ../git-mergetool--lib.sh $(wildcard ../mergetools/*)
+	$(QUIET_GEN)$(RM) $@ && \
+	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && \
+		. ../git-mergetool--lib.sh && \
+		show_tool_names can_diff "* " || :' >mergetools-diff.txt && \
+	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && \
+		. ../git-mergetool--lib.sh && \
+		show_tool_names can_merge "* " || :' >mergetools-merge.txt && \
+	date >$@
+
 clean:
 	$(RM) *.xml *.xml+ *.html *.html+ *.1 *.5 *.7
 	$(RM) *.texi *.texi+ *.texi++ git.info gitman.info
 	$(RM) *.pdf
 	$(RM) howto-index.txt howto/*.html doc.dep
 	$(RM) technical/api-*.html technical/api-index.txt
-	$(RM) $(cmds_txt) *.made
+	$(RM) $(cmds_txt) $(mergetools_txt) *.made
 	$(RM) manpage-base-url.xsl
 
 $(MAN_HTML): %.html : %.txt

Documentation/diff-config.txt

@@ -132,9 +132,10 @@ diff.<driver>.cachetextconv::
 	conversion outputs.  See linkgit:gitattributes[5] for details.
 
 diff.tool::
-	The diff tool to be used by linkgit:git-difftool[1].  This
-	option overrides `merge.tool`, and has the same valid built-in
-	values as `merge.tool` minus "tortoisemerge" and plus
-	"kompare".  Any other value is treated as a custom diff tool,
-	and there must be a corresponding `difftool.<tool>.cmd`
-	option.
+	Controls which diff tool is used by linkgit:git-difftool[1].
+	This variable overrides the value configured in `merge.tool`.
+	The list below shows the valid built-in values.
+	Any other value is treated as a custom diff tool and requires
+	that a corresponding difftool.<tool>.cmd variable is defined.
+
+include::mergetools-diff.txt[]

Documentation/merge-config.txt

@@ -52,12 +52,12 @@ merge.stat::
 	at the end of the merge.  True by default.
 
 merge.tool::
-	Controls which merge resolution program is used by
-	linkgit:git-mergetool[1].  Valid built-in values are: "araxis",
-	"bc3", "diffuse", "ecmerge", "emerge", "gvimdiff", "kdiff3", "meld",
-	"opendiff", "p4merge", "tkdiff", "tortoisemerge", "vimdiff"
-	and "xxdiff".  Any other value is treated is custom merge tool
-	and there must be a corresponding mergetool.<tool>.cmd option.
+	Controls which merge tool is used by linkgit:git-mergetool[1].
+	The list below shows the valid built-in values.
+	Any other value is treated as a custom merge tool and requires
+	that a corresponding mergetool.<tool>.cmd variable is defined.
+
+include::mergetools-merge.txt[]
 
 merge.verbosity::
 	Controls the amount of output shown by the recursive merge

git-mergetool--lib.sh

@@ -1,6 +1,7 @@
 #!/bin/sh
 # git-mergetool--lib is a library for common merge tool functions
-MERGE_TOOLS_DIR=$(git --exec-path)/mergetools
+
+: ${MERGE_TOOLS_DIR=$(git --exec-path)/mergetools}
 
 mode_ok () {
 	if diff_mode