How does bash knows to read autocomple from /usr/lib/git-core/ - linux

Question

I installed git-flow, which copied some bash scripts to /usr/lib/git-core/ which are now available in bash complete.
I know that bash reads the following directories looking for autocomplete scripts:
/etc/bash_completion.d/
/usr/share/bash-completion/
What makes bash aware to those new scripts?
I installed git-flow from the debian repositories, and I am trying to understand what makes this package ticks.
I did not succeed in finding where bash determines it needs to read this directory. Looking through /usr/share/bash-completion/completions/git did not help either.
update
Looking around, it seems that /usr/git-core/git and /usr/bin/git are the same binary file:
~ $ file /usr/bin/git
/usr/bin/git: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=4f634dadca99fb29ed72568d5604df087d6e6502, stripped
~ $ file /usr/lib/git-core/git
/usr/lib/git-core/git: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=4f634dadca99fb29ed72568d5604df087d6e6502, stripped

Answer 1

OK, given the hint from Etan, it seems, that what seems to be a bash completion is actually a git completion. As far why /usr/lib/git-core is search, strace shows that it's hard coded when configuring and building the package:
execve("/usr/bin/git", ["git", "help", "-a"], [/* 73 vars */]) = 0
brk(0) = 0xab4000
...
openat(AT_FDCWD, "/usr/lib/git-core", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 169 entries */, 32768) = 6192
stat("/usr/lib/git-core/git-fetch-pack", {st_mode=S_IFREG|0755, st_size=1577256, ...}) = 0
stat("/usr/lib/git-core/git-web--browse", {st_mode=S_IFREG|0755, st_size=4398, ...}) = 0
stat("/usr/lib/git-core/git-upload-archive", {st_mode=S_IFREG|0755, st_size=1577256, ...}) = 0
stat("/usr/lib/git-core/git-credential-store", {st_mode=S_IFREG|0755, st_size=765192, ...}) = 0
...
When compiling the package from source with configure --prefix=/usr/local/bin/git the strace log is:
execve("/usr/local/git/bin/git", ["/usr/local/git/bin/git", "help", "-a"], [/* 73 vars */]) = 0
brk(0) = 0x2395000
...
openat(AT_FDCWD, "/usr/local/git/libexec/git-core", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 159 entries */, 32768) = 5792
stat("/usr/local/git/libexec/git-core/git-fetch-pack", {st_mode=S_IFREG|0755, st_size=8448439, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-web--browse", {st_mode=S_IFREG|0755, st_size=4398, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-upload-archive", {st_mode=S_IFREG|0755, st_size=8448439, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-credential-store", {st_mode=S_IFREG|0755, st_size=4146627, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-p4", {st_mode=S_IFREG|0755, st_size=122122, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-merge-tree", {st_mode=S_IFREG|0755, st_size=8448439, ...}) = 0
stat("/usr/local/git/libexec/git-core/git-mergetool", {st_mode=S_IFREG|0755, st_size=8377, ...}) = 0

Answer 2

The need to read /etc/bash_completion.d is built into bash which sources all scripts contained within this directory making any defined functionality available.
Each time you start a new terminal, this directory is re-read as part of the bash warmup procedure making all defined completions available to the shell.
Different vendors may have different directories read alongside /etc/bash_completion.d including /usr/share/bash-completion but this one is the minimum defined.
If you want to define your own completions, have a look at https://www.gnu.org/software/bash/manual/html_node/Programmable-Completion.html.