I have an aliases.sh file modified with

alias gc='git checkout'

and upon checking out a long branch name, if I type
gc <branchstring> + TAB , auto-complete doesn't work, in order for the full branch name to appear.

I looked through everything in this thread to try to make it work for & tailor what kpsfoo said, but for the Windows 10 OS.

The steps would be to:

1) Copy the git-completion.bash file from

<your git install folder>/etc/git-completion.bash

to

C:Users<YourUserName>git-completion.bash

2) add this line of code:

source ~/git-completion.bash to your aliases.sh file

(which can be found in <your git install folder>etcprofile.d )

3) Add alias gc='git checkout'
&
Add __git_complete gco _git_checkout anywhere after the source ~/git-completion.bash line in your aliases.sh file.

4) Reboot your git bash and enjoy your alias auto completion!

Example:
If I have a branch VeryVeryLongBranchName and I'm currently on dev branch, and want to switch to it, instead of typing
git checkout VeryVeryLongBranchName I can type only
gc Very +TAB key and it is the equivalent of the instruction above.

An example of everything that I have in my aliases.sh file (and it will be expanded, as I find the need for other aliases) would be:

alias ga="git add"

alias gb='git branch'

alias gba="git branch -a"

alias gc='git checkout'

alias gcb='git checkout -b'

alias gcam='git commit -a -m'

alias gm='git merge --no-ff'

alias gps='git push wpdev dev'

alias gpsm='git push wpdev master'

alias gpl='git pull wpdev dev'

alias gplm='git pull wpdev master'

alias st='git status'

alias l='git log --graph --pretty='''%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset''' --abbrev-commit'

alias last='log -1 HEAD'

alias gs='git stash'



# Enable the __git_complete function to autocomplete aliases once you press TAB

source ~/git-completion.bash



__git_complete ga _git_add

__git_complete gc _git_checkout

__git_complete gm _git_merge

__git_complete gb _git_branch

__git_complete gba _git_branch

__git_complete l _git_log



case "$TERM" in

xterm*)

    # The following programs are known to require a Win32 Console

    # for interactive usage, therefore let's launch them through winpty

    # when run inside `mintty`.

    for name in node ipython php php5 psql python2.7

    do

        case "$(type -p "$name".exe 2>/dev/null)" in

        ''|/usr/bin/*) continue;;

        esac

        alias $name="winpty $name.exe"

    done

    ;;

esac

-worth of note: alias gm='git merge --no-ff' goes just fine with __git_complete gm _git_merge (when typing gm plus a string from your branch name and pressing TAB, it will autocomplete, and, after you run the command, the merge will take into consideration the --no-ff rule)