In git-difftool.txt, it says

        'git difftool' falls back to 'git mergetool' config variables when the
        difftool equivalents have not been defined.

However, when `diff.guitool` is missing, it doesn't fallback to
anything. Make git-difftool fallback to `merge.guitool` when `diff.guitool` is
missing.

Signed-off-by: Denton Liu <liu.den...@gmail.com>
---
 Documentation/git-difftool.txt |  4 +++-
 builtin/difftool.c             | 10 ++--------
 t/t7800-difftool.sh            | 16 ++++++++++++++++
 3 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index 96c26e6aa8..484c485fd0 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -90,7 +90,9 @@ instead.  `--no-symlinks` is the default on Windows.
        When 'git-difftool' is invoked with the `-g` or `--gui` option
        the default diff tool will be read from the configured
        `diff.guitool` variable instead of `diff.tool`. The `--no-gui`
-       option can be used to override this setting.
+       option can be used to override this setting. If `diff.guitool`
+       is not set, we will fallback in the order of `merge.guitool`,
+       `diff.tool`, `merge.tool` until a tool is found.
 
 --[no-]trust-exit-code::
        'git-difftool' invokes a diff tool individually on each file.
diff --git a/builtin/difftool.c b/builtin/difftool.c
index 65bba90338..10660639c0 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -24,7 +24,6 @@
 #include "object-store.h"
 #include "dir.h"
 
-static char *diff_gui_tool;
 static int trust_exit_code;
 
 static const char *const builtin_difftool_usage[] = {
@@ -34,11 +33,6 @@ static const char *const builtin_difftool_usage[] = {
 
 static int difftool_config(const char *var, const char *value, void *cb)
 {
-       if (!strcmp(var, "diff.guitool")) {
-               diff_gui_tool = xstrdup(value);
-               return 0;
-       }
-
        if (!strcmp(var, "difftool.trustexitcode")) {
                trust_exit_code = git_config_bool(var, value);
                return 0;
@@ -734,8 +728,8 @@ int cmd_difftool(int argc, const char **argv, const char 
*prefix)
        if (use_gui_tool + !!difftool_cmd + !!extcmd > 1)
                die(_("--gui, --tool and --extcmd are mutually exclusive"));
 
-       if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
-               setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
+       if (use_gui_tool)
+               setenv("GIT_MERGETOOL_GUI", "true", 1);
        else if (difftool_cmd) {
                if (*difftool_cmd)
                        setenv("GIT_DIFF_TOOL", difftool_cmd, 1);
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 107f31213d..ae90701a12 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -279,11 +279,27 @@ test_expect_success 'difftool + mergetool config 
variables' '
        echo branch >expect &&
        git difftool --no-prompt branch >actual &&
        test_cmp expect actual &&
+       git difftool --gui --no-prompt branch >actual &&
+       test_cmp expect actual &&
 
        # set merge.tool to something bogus, diff.tool to test-tool
        test_config merge.tool bogus-tool &&
        test_config diff.tool test-tool &&
        git difftool --no-prompt branch >actual &&
+       test_cmp expect actual &&
+       git difftool --gui --no-prompt branch >actual &&
+       test_cmp expect actual &&
+
+       # set merge.tool, diff.tool to something bogus, merge.guitool to 
test-tool
+       test_config diff.tool bogus-tool &&
+       test_config merge.guitool test-tool &&
+       git difftool --gui --no-prompt branch >actual &&
+       test_cmp expect actual &&
+
+       # set merge.tool, diff.tool, merge.guitool to something bogus, 
diff.guitool to test-tool
+       test_config merge.guitool bogus-tool &&
+       test_config diff.guitool test-tool &&
+       git difftool --gui --no-prompt branch >actual &&
        test_cmp expect actual
 '
 
-- 
2.21.0.1000.g11cd861522

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