How to refer to a previous commit in git commit message

Question

Is there a convention for referring to a previous commit in a git commit message?

Example commit message:

Fixed bug such and such introduced in a1b2c3e4 

In particular, is there a convention that github.com will understand, and convert to a link?

Answer 1

Yup - GitHub will pick up references to SHAs and users/repos patterns using the GitHub Flavored Markdown

Specifically about linking to commits:

A bit of the GitHub spice

In addition to the changes in the previous section, certain references are auto-linked:

• SHA: be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
• User@SHA ref: mojombo@be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
• User/Project@SHA: mojombo/god@be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
• #Num: #1
• User/#Num: mojombo#1
• User/Project#Num: mojombo/god#1

Answer 2

Guys at git answers the question this way:

If you want to reference a previous commit in the history of a stable branch, use the format "abbreviated hash (subject, date)", like this:

Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30) noticed that ... 

The "Copy commit summary" command of gitk can be used to obtain this format (with the subject enclosed in a pair of double-quotes), or this invocation of git show:

git show -s --pretty=reference <commit> 

or, on an older version of Git without support for --pretty=reference:

git show -s --date=short --pretty='format:%h (%s, %ad)' <commit>