Complete custom git commands in $PATH

Git treats executables in $PATH that start with "git-" as custom
subcommands. Add completion support for them.

Fixes #1680.

share/completions/git.fish

@@ -91,6 +91,24 @@ function __fish_git_aliases
     command git config --get-regexp '^alias\.' | sed -n "s/^alias\.\([^ ]*\).*/\1/p"
 end
 
+function __fish_git_custom_commands
+    # complete all commands starting with git-
+    # however, a few builtin commands are placed into $PATH by git because
+    # they're used by the ssh transport. We could filter them out by checking
+    # if any of these completion results match the name of the builtin git commands,
+    # but it's simpler just to blacklist these names. They're unlikely to change,
+    # and the failure mode is we accidentally complete a plumbing command.
+    set -l IFS \n
+    for name in (builtin complete -Cgit- | sed 's/^git-\([^[:space:]]*\).*/\1/')
+        switch $name
+            case cvsserver receive-pack shell upload-archive upload-pack
+                # skip these
+            case \*
+                echo $name
+        end
+    end
+end
+
 # general options
 complete -f -c git -n 'not __fish_git_needs_command' -l help -d 'Display the manual of a git command'
 
@@ -392,3 +410,6 @@ complete -f -c git -n '__fish_git_needs_command' -a whatchanged -d 'Show logs wi
 
 ## Aliases (custom user-defined commands)
 complete -c git -n '__fish_git_needs_command' -a '(__fish_git_aliases)' -d 'Alias (user-defined command)'
+
+## Custom commands (git-* commands installed in the PATH)
+complete -c git -n '__fish_git_needs_command' -a '(__fish_git_custom_commands)' -d 'Custom command'