In git-difftool, if the tool is called with --gui but `diff.guitool` is
not set, it falls back to `diff.tool`. Make git-mergetool also fallback
from `merge.guitool` to `merge.tool` if the former is undefined.

If git-difftool were to use `get_configured_mergetool`, it would also
get the fallback behaviour in the following precedence:

1. diff.guitool
2. merge.guitool
3. diff.tool
4. merge.tool

Signed-off-by: Denton Liu <liu.den...@gmail.com>
---
 Documentation/git-mergetool.txt |  4 +++-
 git-mergetool--lib.sh           | 27 ++++++++++++++++++---------
 t/t7610-mergetool.sh            | 19 +++++++++++++++++++
 3 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/Documentation/git-mergetool.txt b/Documentation/git-mergetool.txt
index 0c7975a050..6b14702e78 100644
--- a/Documentation/git-mergetool.txt
+++ b/Documentation/git-mergetool.txt
@@ -83,7 +83,9 @@ success of the resolution after the custom tool has exited.
 --gui::
        When 'git-mergetool' is invoked with the `-g` or `--gui` option
        the default merge tool will be read from the configured
-       `merge.guitool` variable instead of `merge.tool`.
+       `merge.guitool` variable instead of `merge.tool`. If
+       `merge.guitool` is not set, we will fallback to the tool
+       configured under `merge.tool`.
 
 --no-gui::
        This overrides a previous `-g` or `--gui` setting and reads the
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index d5e2c6c5c6..68a85f4a7b 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -350,20 +350,29 @@ guess_merge_tool () {
 }
 
 get_configured_merge_tool () {
-       # If first argument is true, find the guitool instead
-       if test "$1" = true
+       is_gui="$1"
+       sections="merge"
+       keys="tool"
+
+       if diff_mode
        then
-               gui_prefix=gui
+               sections="diff $sections"
        fi
 
-       # Diff mode first tries diff.(gui)tool and falls back to 
merge.(gui)tool.
-       # Merge mode only checks merge.(gui)tool
-       if diff_mode
+       if "$is_gui" = true
        then
-               merge_tool=$(git config diff.${gui_prefix}tool || git config 
merge.${gui_prefix}tool)
-       else
-               merge_tool=$(git config merge.${gui_prefix}tool)
+               keys="guitool $keys"
        fi
+
+       IFS=' '
+       for key in $keys
+       do
+               for section in $sections
+               do
+                       merge_tool=$(git config $section.$key) && break 2
+               done
+       done
+
        if test -n "$merge_tool" && ! valid_tool "$merge_tool"
        then
                echo >&2 "git config option $TOOL_MODE.${gui_prefix}tool set to 
unknown tool: $merge_tool"
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 5f37d7a1ff..bc2c9eaa30 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -167,6 +167,25 @@ test_expect_success 'gui mergetool' '
        git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'gui mergetool without merge.guitool set fallsback to 
merge.tool' '
+       test_when_finished "git reset --hard" &&
+       git checkout -b test$test_count branch1 &&
+       git submodule update -N &&
+       test_must_fail git merge master >/dev/null 2>&1 &&
+       ( yes "" | git mergetool --gui both >/dev/null 2>&1 ) &&
+       ( yes "" | git mergetool -g file1 file1 ) &&
+       ( yes "" | git mergetool --gui file2 "spaced name" >/dev/null 2>&1 ) &&
+       ( yes "" | git mergetool --gui subdir/file3 >/dev/null 2>&1 ) &&
+       ( yes "d" | git mergetool --gui file11 >/dev/null 2>&1 ) &&
+       ( yes "d" | git mergetool --gui file12 >/dev/null 2>&1 ) &&
+       ( yes "l" | git mergetool --gui submod >/dev/null 2>&1 ) &&
+       test "$(cat file1)" = "master updated" &&
+       test "$(cat file2)" = "master new" &&
+       test "$(cat subdir/file3)" = "master new sub" &&
+       test "$(cat submod/bar)" = "branch1 submodule" &&
+       git commit -m "branch1 resolved with mergetool"
+'
+
 test_expect_success 'mergetool crlf' '
        test_when_finished "git reset --hard" &&
        # This test_config line must go after the above reset line so that
-- 
2.21.0.967.gf85e14fd49

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