[PATCH] mergetool--lib: fix '--tool-help' to correctly show available tools

From: Philippe Blain <philippe.blain@canada.ca>

Commit 83bbf9b92e (mergetool--lib: improve support for vimdiff-style tool variants, 2020-07-29) introduced a regression in the output of `git mergetool --tool-help` and `git difftool --tool-help` [1].

In function 'show_tool_names' in git-mergetool--lib.sh, we loop over the supported mergetools and their variants and accumulate them in the variable 'variants', separating them with a literal '\n'.

The code then uses 'echo $variants' to turn these '\n' into newlines, but this behaviour is not portable, it just happens to work in some shells, like dash(1)'s 'echo' builtin.

For shells in which 'echo' does not turn '\n' into newlines, the end result is that the only tools that are shown are those that are found and have variants, since the variants are separated by actual newlines in '$variants' because of the several 'echo' calls in mergetools/{bc,vimdiff}::list_tool_variants.

Fix this bug by embedding an actual line feed into `variants` in show_tool_names(). While at it, replace `sort | uniq` by `sort -u`.

To prevent future regressions, add a simple test that counts the number of tools shown by 'git mergetool --tool-help', irrespective of their installed status, by making use of the fact that mergetools are listed by 'git mergetool --tool-help' on lines starting with tabs. Prefix the `git config` commands used at the beginning of the test to remove the fake tools used by the previous test with 'test_might_fail' so that the test can be run independantly of the previous test without failing.

[1] https://lore.kernel.org/git/CADtb9DyozjgAsdFYL8fFBEWmq7iz4=prZYVUdH9W-J5CKVS4OA@mail.gmail.com/

Reported-by: Philippe Blain <levraiphilippeblain@gmail.com>
Based-on-patch-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
    Fix regression in 'git {diff,merge}tool --tool-help'
    
    I went with Johannes' suggestion finally because upon further
    inspection, René's patch for some reason (I did not debug further)
    caused to code to never reach 'any_shown=yes' in show_tool_help,
    therefore changing the output of the command.
    
    I guess it's too late for inclusion in 2.30...

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-825%2Fphil-blain%2Fmergetool-tool-help-fix-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-825/phil-blain/mergetool-tool-help-fix-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/825

 git-mergetool--lib.sh |  6 ++++--
 t/t7610-mergetool.sh  | 10 ++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)